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The risks associated with post term pregnancy: a literature review
Aust. Mid. J. ACM 2005, 18 (2): 10-16 New Scholar
The risks associated with post term pregnancy: a literature review Juliana Brennan RN, Grad Dip Midwifery, M Mid
Angliss Health Service Family Birth Centre, Ferntree Gully, Victoria, Australia Correspondence Juliana Brennan Email julianabrennan @optusnet.com.au
Abstract The purpose of this paper is to conduct a critical literature review of the risks associated with induction of labour and a conservative approach to post term pregnancy. The main aim was to establish whether a conservative approach to post term pregnancy is associated with increased rates of perinatal mortality and morbidity, and whether induction of labour reduces these rates. Electronic databases and texts were examined. The findings were that the rates of caesarean section, instrumental birth, use of analgesia, incidence of fetal heart rate abnormality, meconium aspiration syndrome and fetal size were similar in both approaches to care. It appears that perinatal mortality rates increase in post term pregnancy yet the literature varies as to when this increase becomes significant. Induction of labour after 41 weeks gestation reduces the rates of perinatal mortality, however, the amount to which mortality rates are decreased by performing induction of labour at this gestation also varies within the literature. Therefore, it is difficult to give concise dates about when induction of labour should be recommended. Women should be informed of the risks associated with both approaches to care, and based on the review findings, they should be offered induction of labour between 291 days and 294 days, or between 41+4 and 41+7 weeks gestation. However, their preference for either approach should be respected.
Introduction This paper presents a critical review of the research literature on the risks associated with post term pregnancy. The purposes of this paper are first, to establish whether perinatal mortality and morbidity rates increase in pregnancy advancing beyond term, and whether induction of labour decreases the rates of perinatal mortality. Second, the paper seeks to utilise the information gathered from the critical review to develop recommendations for midwives to use when informing women about the risks associated with post term pregnancy. Background The definition of post term pregnancy is contested by some within the literature who argue that post term pregnancy is pregnancy of greater than 287 days, or 41 completed weeks of gestation (Halliday & Riley, 2003; Goeree et al., 1995; The National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units (NICH) et al., 1994; Keirse, 1993). The internationally recognised definition of post term pregnancy, however, is 42 completed weeks, or 294 days or more from the first day of the last menstrual period (Cunningham et al., 1997). Many research studies and texts also support this definition (Calder et al., 2001; Dublin, et al., 2000; Enkin et aL, 2000; World Health Organisation, 1996; Alfirevic & Walkinshaw, 1995; Sims & Walther, 1989; Augensen et al., 1987). The variations in definitions has led to different timings of when labour is induced in the induction arms of the Randomised Controlled Trials (RCT) comparing a conservative approach and induction of labour. This has made the interpretation of the optimum duration of pregnancy difficult (Gulmezoglu & Crowther, 2004). Furthermore, the various terms used to describe gestation are often unclear and confusing. The most recent Cochrane review assessing the effects of interventions for preventing or 10
improving the outcome of delivery at or beyond term states that routine induction of labour "after 41 weeks gestation" appears to reduce perinatal mortality rates (Crowley, 2002, p.1). The term "after 41 weeks" is ambiguous and vague. Does this mean that any time from 41 completed weeks gestation is an acceptable time to recommend induction of labour? It can only be assumed that this means any time after 41 completed weeks gestation, or after 287 days, and for clarity purposes, it would have been less ambiguous to have used the term "from 41 completed weeks gestation", or "from 287 days". It appears that perinatal mortality rates increase with post term pregnancy, however, there are conflicting views as to when these increases become significant. A retrospective study conducted by Hilder et al., (1998), shows that when stillbirth, neonatal, and post neonatal (infant losses up to one year of life) mortality rates are combined, they increase significantly in term and post term pregnancy (see Table 1 and 2). A cross-sectional study by Olesen et aL, (2003) has found that perinatal mortality rates significantly increase from 42 completed weeks gestation, or 294 days. However, some individual RCTs have found no significant differences in perinatal mortality rates after 41 completed weeks gestation, or from 287 days (Hannah et al., 1996; The National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units (NICH) et al., 1994; Cardozo, 1993; Hannah et al., 1992). A quality cohort study has found no significant increase in perinatal mortality rates even after 42 completed weeks, or 294 days (Campbell et al., 1997). A Cochrane systematic review provides evidence that 'routine' induction of labour after 41 weeks gestation reduces the risk of perinatal death in normally formed babies (OR 0.23; 95% CI 0.06 to 0.90) (Crowley, 2002). However, another systematic review performed by Sanchez-Ramos et al., (2003) has found that induction of labour at or just after 41 weeks gestation does not significantly Australian Midwifery Journal 2005, 18 (2)
Juliana Brennan
reduce perinatal mortality rates (OR 0.41; 95% CI 0.14 to 1.18). It has also been suggested that induction of labour may be associated with an increase in perinatal morbidity, especially when the cervix is not favourable for induction (Gulmezoglu & Crowther, 2004; Duff et al, 2000). However, a conservative approach has also been identified as being associated with an increase in perinatal morbidity (Gulmezoglu & Crowther, 2004; Olesen et al., 2003; Sanchez-Ramos, et al., 2003; Campbell et aL, 1997). These variations in findings have led to much controversy and confusion over how women with post term pregnancy should be cared for. Therefore, conducting a critical review of the literature was important. It has allowed for the clarification of definitions and terms used to describe post term pregnancy. Importantly, practice recommendations have been developed providing midwives with information to share with women based on the best available evidence. The reported frequency of pregnancy extending beyond 42 completed weeks gestation (294 days) varies from 3.5% to 14% (Menticoglou & Hall, 2002; Enkin et al., 2000; Hilder et aL, 1998; Sims & Walther, 1989; Cardozo et al., 1986). In the State of Victoria, pregnancy advancing beyond 41 weeks gestation has steadily declined from 4% in 1986 to 1.2% in 2002. This may be due to more accurate pregnancy dating through the use of early pregnancy ultrasound (Enkin et al., 2000), however, the increasing rates of inductions of labour are probably the most likely cause of this decline. Post term pregnancy, defined by Halliday and Riley (2003) as pregnancy advancing beyond 41 completed weeks (287 days) gestation, was the single most common reason for inducing labour in 2002 in the State of Victoria, and 24.2% of all inductions were performed as a result of pregnancy extending beyond this gestation. It has been suggested that the most frequent cause in apparent post term pregnancy is an error in pregnancy dating (Crowley, 2002; Neilson, 2003). These errors may be reduced by the use of early ultrasound in pregnancy (Neilson, 2003). Although the cause of truly post term pregnancy is unknown (Alfirevic & Walkinshaw, 1994), there are identifiable risk factors for post term pregnancy. Primiparity is the most common risk factor (Campbell et al., 1997; Alfirevic & Walkinshaw, 1994), and there may be a genetic predisposition for some women to have more prolonged pregnancies than others (Olesen et al., 2003; Campbell et aL, 1997) Fetal chromosomal abnormalities are also associated with post term pregnancies (Hilder et al., 1998; Campbell et al., 1997).
Literature search strategy A comprehensive literature review was performed using text books, Medline, Pub Med, Cinahl, MIDIRS and the Cochrane Database. Search headings included; ! Risks associated with induction of labour versus a conservative approach to post term pregnancy I Maternal morbidity associated with induction versus a conservative approach which included instrumental vaginal birth, perineal trauma, caesarean section rates, narcotic analgesia, and epidural anaesthesia. i Neonatal morbidity associated with induction versus a conservative approach which included meconium liquor, meconium aspiration, neonatal seizures, asphyxia, birth injuries, low 5 minute apgar scores, incidence of intubation, and incidence of fetal heart rate abnormality. ! Perinatal mortality. Australian Midwifery Journal 2005, 18 (2)
Overall, a range of Systematic reviews, RCTs, Cohorts, retrospective, observational, qualitative studies, reviews and commentaxies were obtained. All were of varying quality, date ranges between 1969 and 2004. There have been very few published RCTs since 1992 when Hannah et al., (1992) conducted a RCT which methodologically is considered a high quality study (Grant, 1994). The articles were mainly in the medical literature, with limited research conducted by midwives. Articles published in languages other than English were excluded from review.
Risks associated with induction of labour Findings in relation to risks associated with induction of labour compared with a conservative approach for the care of women with post term pregnancy vary within the literature. Results from a retrospective study suggests that induction of labour has certain risks to both mother and baby when compared with a conservative approach to post term pregnancy. Some of the reported risks to the mother of induction of labour include increased rates of mean average blood loss, caesarean section and increased use of epidural during labour (Duff et al., 2000). Risks to the fetus and neonate include the possibility of fetal distress, bruising, and increased incidence of lower apgar scores (Duff et al., 2000). Some individual RCTs also found increased risk associated with induction of labour compared with a conservative approach to post term pregnancy (Augensen, et al. 1987; Cardozo et al., 1986). Induction of labour has been found to be associated with an increase in perineal trauma, fetal distress, lower apgar scores, higher rates of intubation of neonates (Cardozo, 1986), and an increase in the incidence of neonatal jaundice (Augensen et aL, 1987). However, a Cochrane systematic review conducted by Crowley, (2002) evaluated the effect of induction of labour on various maternal and neonatal morbidities in post term pregnancy. The review found that induction of labour after 41 weeks gestation had no statistically significant effect on instrumental birth rate, use of analgesia, or incidence of fetal heart rate abnormality when compared with a conservative approach to post term pregnancy. These findings, however, may not be valid because all the RCTs used in the systematic review were subject to the possibility of detection bias, as blinding as to mode of onset of labour was impossible (Crowley, 2002). Therefore, firm conclusions cannot be made regarding instrumental birth rate findings (Crowley, 2002). Nevertheless, this is the best available evidence on the effect of induction of labour on maternal and neonatal morbidity in post term pregnancy to date. There are widely held beliefs that induction of labour for post term pregnancy is associated with higher caesarean section rates (Duff et al., 2000; Enkin, 2000). The Canadian Multi-Centre Post term Pregnancy Trial conducted by Hannah et aL, (1992) claims that induction of labour beyond 41 weeks gestation (from 287 days) was, in fact, more likely to reduce the rate of caesarean section than a conservative approach to post term pregnancy. This finding was also made in a systematic review by Sanchez-Ramos et aL, (2003). However, the validity of the findings of the Canadian Multi-Centre Post term Pregnancy Trial in relation to the caesarean section rates have been criticised (Crowley, 2002; Mentacoglou & Hall, 2002; Keirse, 1993). One major problem with the RCT conducted by Hannah (1992) according to Keirse (1993) is that endocervical administration of prostaglandin (PGE2) was routinely available to all women assigned to the induction group, but was not part of the policy if induction was performed in the conservative approach group. According to Keirse (1993), 77.2% of the induction group 11
Juliana Brennan
Table 1 Prospective risk of stillbirth (SB), neonatal (NN) and post neonatal (PNN) death and pregnancy loss rates per 1000 ongoing pregnancies (% op) at each week of gestation (statistics from Hilder et aL (2000) and Hilder et aL (1998)
Gestation (weeks) 35 36 37 38 39 40 41 42 43
No of OP
No of SB
161 638 159 723 155 791 147 631 126 448 93 539 39 245 10 305 1 874
48 62 47 77 62 81 50 16 4
Risk of SB/1000 OP (95% Cl) 0.30 0.39 0.30 0.52 0.49 0.87 1.27 1.55 2.13
Risk of SB in ensuing week
(0.23-0.37) (0.31-0.46) (0.23-0.37) (0.44-0.60) (0.40--0.58) (0.80-0.96) (0.94-1.60) (0.93-2.78) (0.28-3.99)
received PGE2, compared with only 27.7% of those women from the conservative approach group who were induced. It has been reported in some literature that the incidence of operative delivery overall is lower in women induced with prostglandin (Enkin et al., 2000). Therefore, there is likely to be a difference in caesarean section rate in the study by Hannah et al., (1992), as the conservative management group were not offered the best treatment option (Keirse, 1993). In argument against this finding, the Consultative Council on Obstetric and Paediatric Mortality and Morbidity (2003) states that the use of PGE2 has been associated with an increased rate of fetal death, and recommends exercising care when using prostglandins for women with an unfavourable cervix. Yet no research evidence has been found to support this claim. Yogev et al., (2003) states that no serious complications occur with the use of PGE2 in women with post term pregnancy (of more than 294 days) and Misra and Vavre (1994) state that perinatal mortality is not affected when comparing the use of PGE2 and oxytocin in women with a very unfavourable cervix. The Cochrane Review states that routine induction of labour for post term pregnancy is not associated with an increased risk of caesarean section, regardless of parity, state of the cervix, or method of induction. The rates of caesarean section are similar among both approaches to care (Crowley, 2002). C o n s e r v a t i v e a p p r o a c h a n d perinatal m o r b i d i t y
Most of the literature confirms that there is an increase in the incidence of meconium stained amniotic fluid in pregnancies extending beyond 41 weeks (from 287 days), (Enkin et al., 2000; Crowley, 2002; Grant, 1994; Augensen et al., 1987). However, although there is an increased incidence of meconium stained amniotic fluid, it is well documented that induction of labour does not reduce the risk of meconium aspiration syndrome. (SanchezRamos et al., 2003; Crowley, 2002; Enkin et al., 2000; Grant, 1994) or neonatal seizures (Crowley 2002; Enkin et al., 2000). Therefore, it is questionable whether induction of labour performed for this reason is beneficial. A cross sectional study by Olesen et al., (2003) has found there to be significantly increased risks of maternal complications such as post partum haemorrhage, cephalopelvic disproportion, cervical rupture, shoulder dystocia and puerperal infection when pregnancy extends beyond 42 completed weeks gestation, or 294 days (Olesen et al., 2003). When the risk of maternal and fetal complications were stratified on birthweight, it was found that there were more maternal complications with birthweights < 2500 g and with birthweights > 4500 g than in birthweights of 2500--4499g (Olesen et al., 2003). 12
1:3332 1:2536 1:3332 1:1922 1:2039 1:1148 1:786 1:644 1:486
Stillbirth rate (% OP) 0.33 0.45 0.35 0.56 0.57 0.86 1.27 1.55 2.12
NN & PNN Mortality Rate (% OP)
Total pregnancy loss rate (% OP)
0.16 0.38 0.34 0.70 0.83 1.57 1.48 3.29 3.71
0.5 0.8 0.7 1.3 1.4 2.4 2.8 4.8 5.8
A Cohort study conducted by Campbell et al., (1997) also states that maternal complications were generally associated with larger fetal size (LGA) (defined as birthweight equal or greater than the 90th percentile) (Campbell et al., 1997). Labour dysfunction and obstetric trauma were associated with both LGA and post term birth, however shoulder dystocia was associated with LGA only. The RCTs to date do not appear to have examined matemal complications of post term pregnancy, other than caesarean section, instrumental birth rates and use of analgesia. However, there are limited numbers of RCTs that have examined fetal size between induction of labour and conservative approach groups. These have stated that no differences in birthweights were found between groups (NICH et al., 1994; Egarter, 1989; Cardozo, 1986). Therefore, the best available evidence suggests that induction of labour does not have an effect on fetal size when compared with a conservative approach to post term pregnancy. Perinatal m o r t a l i t y rates a n d post t e r m p r e g n a n c y
It is stated in the literature that there are increased risks of perinatal mortality as pregnancy advances beyond 42 weeks gestation, or 294 days (Gulzemoglu & Crowther, 2004; Olesen et al., 2003). However, there is also literature to support that perinatal mortality rates increase prior to this gestation (Hilder et aL, 1998). In a retrospective analysis of 171,527 births in the North East Thames region of London between 1989-1991, it has been found that the rate of stillbirth, expressed as ongoing pregnancies, increases slightly in each ensuing week at and beyond 280 days (see Table 1 & Table 2). Similar rates of increases can be seen in Denmark, Sweden, New Zealand, New York City and England (Olesen et al., 2003; Menticoglou & Hall, 2002; Hilder et al., 1998; Ingemarsson & Kallen, 1997). However, when stillbirth, neonatal and post neonatal mortality rates (infant losses up to one year of age) are examined, there is a significant increase in mortality rates in term and post term pregnancy (see Table 1 & Table 2) (Hilder et al., 1998; Hilder et al., 2000). It is interesting to note that although some of these infant deaths result from perinatal complications, most have no known cause (American College of Obstetricians and Gynaecologists, 2004). One explanation offered by Hilder et al. (1998) as to why there is a significant increase in mortalities among these infants, is that post term pregnancies may well be compromised before birth. If this is the case, then induction of labour may not decrease the rate of infant losses up to one year of age. Induction of labour at an earlier gestation may simply shift these losses from post term births, to term births. One limitation to the retrospective analysis performed by Australian MidwiferyJournal 2005, 18 (2'.
The risks associated with post term pregnancy: a literature review
Table 2 Rate of stillbirth compared with infant losses up to 1 year by gestational age (Statistics adapted from Hilder, et aL (1998) Rate of stillbirth compared with infant losses up to 1 yr by gestational age. Adapted from Hilder et aL, (1998) Total infant losses (up to 1 yr)
._~~'~ 0~ r ~ .go O3r
stillbirths
6 Q" ~
5
t - t~ ..~ O~ t~
4
._ ~
o-~_o ~o-~ ~ c w ~ rr
O e-
9 0 ~ o.o..4
2
0 35
36
37
38
39
40
41
42
43
W e e k s of gestation
Hilder et al., (1998) is that pregnancies with chromosomal abnormalities were not eliminated from the study. It is widely acknowledged that pregnancies affected with chromosomal abnormalities are associated with post term pregnancy (Hilder et aL, 1998; Campbell, 11997). Therefore, this is likely to affect the mortality rates presented in this study. A cross sectional study by Olesen et aL (2003) has found that the risk of perinatal death was significantly higher in pregnancy advancing beyond 42 completed weeks gestation, or 294 days. However, this study acknowledges that the results give no indication as to whether this risk may be reduced by inducing labour before 42 weeks of gestation (before 294 days). A 10-year cohort study was conducted in Norway assessing risk factors and birth outcomes for post term pregnancy (Campbell et al., 1997). The results from this study show that there was only a slightly increased risk of perinatal mortality in post term (294 days or more) as compared with term births. This study included stillbirths and early neonatal losses, of less than 168 hours after birth. According to Campbell et al., (1997), the evidence for an adverse impact on perinatal mortality in post term birth is weak once other factors are taken into account. For post term births (from 294 days), the risk factors were small for gestational age (SGA), defined as birth weight less than the 10th percentile of all births of the same gestational age, and maternal age > 35 years. Large for gestational age (LGA) was a protective factor against perinatal mortality (Campbell et al., 1997). This has important implications for midwifery practice. Several studies have shown quite good sensitivity and specificity of fundal height measurements for predicting low birth weight for gestation (Enkin et al., 2000). Where low birth weight for gestation is suspected, midwives must be aware that this condition may constitute a 'high risk' situation. A Cochrane systematic review performed by Crowley (2002) examined 19 RCTs which showed one fetal death occurring in the induction group, compared with nine fetal deaths in the conservative group (total number of participants 7925). This difference is both clinically important and statistically significant (Enkin et aL, 2000). However, it appears that some of the nine deaths that occurred in the conservative management groups in the RCTs evaluated in the Cochrane review, may have occurred for reasons other than post term pregnancy. Caritis et al., (1995) states that one death occurring in the conservative management group (Cardozo, 1986), was as the result of Australian Midwifery Journal 2005, 18 (2)
acute abruptio placenta which may be a condition unrelated to post term pregnancy. There was also one fetal death due to a cord complication which could have happened at any time of gestation, and according to the author, could not be attributed to post term pregnancy (Egarter et al., 1989). Menticoglou and Hall (2002) adds to the critical review by saying that one further stillbirth occurred in a mother with an abnormal glucose tolerance test (GTT) (in Henry 1969). This stillbirth should have been excluded from the study, as gestational diabetes is not considered to be without increased risk, and this stillbirth may therefore have been due to a cause unrelated to post term pregnancy. Gestational diabetes was a criteria for exclusion from the Hannah (1992) trial. One further death occurred at 42 weeks in a 2600 g infant ascribed to fetal distress which presumably could have occurred and resulted in similar management difficulties during an earlier induction. A 2600 g weight at 42 weeks is a low birth weight, and the hypothesis that this death occurred directly as a result of gestation alone is questionable (Menticoglou & Hall 2002). Campbell et aL (1997) state that poor fetal growth is, indeed, the major risk factor for fetal death, regardless of gestation.
The effect of Induction of labour on perinatal mortality rates There have been numerous research studies comparing outcomes of induction of labour at various gestations with a conservative approach to post term pregnancy. The largest and methodologically most outstanding RCT conducted to date is the Canadian Multi-centre Post term Pregnancy Trial by Hannah et al. (1992). Although this trial has been criticised in relation to the claim that induction of labour reduces the rate of caesarean sections, this trial is not only the largest RCT, but methodologically it is the best of all the RCTs performed to date (Grant, 1994). This trial found that the rates of perinatal mortality and neonatal morbidity were similar with either inducing labour or adopting conservative approaches to the care of women with pregnancy extending beyond 41 weeks gestation (from 287 days). Although a higher rate of perinatal deaths occurred in the conservative approach group, the results were not statistically significant (Hannah et al., 1992). A systematic review conducted by Sanchez-Ramos et al. (2003) reviewed 16 RCTs and compared the outcomes of routine induction of labour and expectant management for uncomplicated pregnancies at and just after 41 weeks gestation. This review included 6588 subjects from 10 countries over 33 years. The review paid particular attention to homogeneity and only included RCTs with uncomplicated pregnancies. Sensitivity analysis revealed that no studies had a disproportionate impact on the outcomes. The findings were that a strategy of routine induction at or just after 41 weeks does not significantly decrease rates of perinatal mortality (OR 0.41; 95%CI 0.14, 1.18). The major weakness of this systematic review was identified by the authors as being a lack of an appropriate comparison group to elective induction at 41 weeks. In four RCTs, the physicians intervened at 43 weeks, in four others at 44 weeks, and two trials did not specify (Sanchez-Ramos et al., 2003). These issues can also be identified as being a limitation to the Cochrane review, as this review, along with the review by Sanchez-Ramos et aL (2003) examined some of the same RCTs. The most recent systematic review performed for Cochrane, comparing a policy of induction at and from > 40 weeks gestation 13
Juliana Brennan
(from 280 days) with a conservative approach, suggest a reduction in perinatal mortality rates with induction of labour after 41 weeks gestation (0.23 95% CI 0.06 to 0.90) (Crowley, 2002). It is interesting to note that some RCTs examined the effect of induction of labour at 40 weeks gestation, or at 280 days. In these trials, there was found to be a significant increase in instrumental vaginal births without any decrease in perinatal mortality rates, therefore induction of labour between 280 days and 286 days is unwarranted (Enkin et al., 2000; Grant, 1994). From the Cochrane systematic review, it has been suggested that approximately 500 inductions would have to be performed after 41 weeks to prevent one perinatal death from occurring (Crowley, 2002). Although, Crowley (2002) also states that the number of inductions required to prevent one perinatal death may be higher in present day practice as these figures may be biased by the high perinatal mortality rate reported in one of the trials used in the systematic review, which was a poor quality study by Hertry (1969). Unfortunately, no systematic reviews have examined the effect of induction of labour on reducing perinatal mortality rates when induction of labour is performed at 42 completed weeks gestation (294 days). Discussion
Cochrane systematic reviews are considered 'gold standard' from which to guide decisions for research and practice (Gulmezoglu & Villar, 2003). However, the methodological quality varies considerably among the 19 RCTs used in the Cochrane Systematic review (Crowley, 2002). In three of the trials used in the systematic review, there are worrying disparities between the numbers in the two arras of the trials (Crowley, 2002). One of the RCTs included in the systematic review (by Martin 1989) only included a total sample size of 22 women (Grant, 1994). Post-randomisation exclusions that cannot be reinstated present a problem in the Martin (1978) trial (Crowley, 2002). Although these problems were identified, these RCTs were not removed from the review. If the Henry (1969), Martin (1978), and Martin (1989) trials were eliminated from the systematic review, the perinatal mortality rates would be reduced by three deaths. This would mean that six deaths would have occurred in the conservative approach group versus one death in the induction of labour group. Based on these figures, approximately 600 inductions would have to be performed after 41 weeks (at and from 287 days) to prevent one perinatal death. Furthermore, if these RCTs, along with the deaths caused by placental abruption (Cardozo, 1986), cord complications (Egarter, 1989), and SGA neonate (Hannah, et al. 1992) were removed from overview, the perinatal mortality rates would be reduced by six deaths. This would mean that three deaths would have occurred in the conservative approach group versus one death in the induction of labour group. Based on these figures, > 1000 inductions would have to be performed after 41 weeks (from 287 days) to prevent one perinatal death. It appears from the literature that perinatal mortality rates increase in post term pregnancy, yet the results vary as to when these findings become significant. The findings also vary as to whether induction of labour significantly reduces perinatal mortality rates if performed at or from 41 weeks gestation. Therefore, it is difficult to give concise dates about when induction of labour should be recommended. However, based on the above information, it would be acceptable to offer women induction of labour from 291 days to 294 days of pregnancy (41+4-41+7). 14
For those women who have no other risk factors, and who wish to adopt a conservative approach to post term pregnancy, they must be informed that perinatal mortality rates appear to increase in pregnancy advancing beyond term, and that induction of labour performed after 41 weeks appears to reduce perinatal mortality rates (Crowley, 2002). It must also be mentioned to women that there has been a considerable amount of research performed in relation to the effectiveness of various methods of fetal surveillance in reducing perinatal mortality and morbidity in post term pregnancy, where a conservative approach rather than induction of labour is adopted. Unfortunately, there is little evidence from RCTs and other studies to suggest that any form of fetal surveillance is reliable in detecting post term pregnancy becoming 'high risk' (Crowley 2002; Alferevic & Walkinshaw 1995; Grant 1994). Nevertheless, women should be informed of the risks associated with both approaches to care, and their preference for either approach should be respected (Hannah et al. 1996). Practice recommendations
Based on the available evidence, women should be informed of the following; 1 The research has shown contradictory findings in relation to the role of induction of labour reducing perinatal mortality rates. The best available evidence suggests that induction of labour some time after, yet not at 41 weeks gestation (287 days), reduces perinatal mortality rates. Therefore, induction of labour should be offered to women between 291 and 294 days (41+4 and 41+7). 2 The review found that induction of labour after 41 weeks gestation had no statistically significant effect on instrumental birth rates, use of analgesia, or incidence of fetal heart rate abnormality when compared with a conservative approach to post term pregnancy. Induction of labour does not appear to have an effect on meconium aspiration syndrome or fetal size. Induction of labour between 280 days and 286 days is associated with significantly higher rates of instrumental vaginal births, without significantly decreasing perinatal mortality rates, and is, therefore, unjustified. 3 Women should be informed that the best available evidence suggests that approximately 500 inductions would have to be performed after 41 weeks to prevent one perinatal death from occurring compared with a conservative approach to post term pregnancy. This figure is likely to be greater than this in present day practice. 4 SGA fetuses, regardless of gestation, and fetuses of women older than 35 years of age are at greater risk of perinatal death. 5 Minimal evidence exists from RCTs and other studies to suggest that any form of fetal surveillance is reliable in detecting post term pregnancy becoming 'high risk'. Conclusion
The findings of this review are, that perinatal mortality rates appear to increase in advancing pregnancy beyond term, and that induction of labour after 41 weeks gestation reduces perinatal mortality rates. Induction of labour for post term pregnancy is not associated with adverse outcomes when compared with a conservative approach to post term pregnancy, and based on these findings, induction of labour should be offered to women between 291 and 294 days gestation (41 + 4 - 41+7). Further research is required into the causes of post term pregnancy, and better methods of identifying the post term fetus at risk of perinatal death. Australian MidwiferyJournal 2005, 18 (2)
The risks associatedwith post term pregnancy:a literature review
The main limitations to this review are that the methodological quality, the definitions of what constitutes post term pregnancy, and the timing for inductions of labour, all varied considerably in the RCTs used in the Cochrane Review by Crowley (2002), possibly making the conclusions difficult to interpret. It is possible that had only high quality, properly conducted RCTs be included in the Cochrane review, that perinatal mortality rates would not be significantly reduced by induction of labour after 41 weeks gestation, and that perinatal mortality rates would only be significantly reduced at a later gestation. Gulmezoglu and Crowther (2004) are conducting a new review for Cochrane, taking into account the limitations of the existing Cochrane review by Crowley (2002). Their hypothesis is that a policy of labour induction at or beyond term compared with a policy of awaiting spontaneous labour indefinitely, improves pregnancy outcomes for the infant and the mother.
Acknowledgement I would like to gratefully acknowledge the assistance provided to me by Kathleen Fahy and Louise Noorbergen. Thank you to my husband Tim, for your support.
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Augensen K Augensen, K. and Bergsjo, P. and Eikeland, T and Askvik, K. and Carlsen, J. (1987) 'Randomised comparison of early versus late induction of labour in post-term pregnancy'. British Medical Journal, 294, 1192-1195.
Henry G. (1969) 'A controlled trial of surgical induction of labour and amnioscopy in the management of prolonged pregnancy'. Journal of Obstetrics and Gynaecology British Commonwealth, 76, 795-798.
Calder, A.A., Beech-Lawrence B., Cookson R., Crowley P., Danielian P., Farebrother A., Foulkes A., Harris P., Lewis G., Neilson J., Rogers J. (2001)'Induction of Labour: Evidence-based Clinical Guideline Number 9'. Clinical Effectiveness Support Unit, Royal College of Obstetricians and Gynaecologists (online). Available: http://www.rcog.org.uk/guidelines.asp?PagelD=108&GuidelinelD=37
Hilder, L. and Costeloe, K. and Thilaganathan, B. (2000) Prospective risk of stillbirth: Study's results are flawed by reliance on cumulative prospective risk. British Medical Journal, 320, 444.
Campbell K.M., Ostbye T., Irgens L.M. (1997)'Post-Term Birth: Risk Factors and Outcomes in a 10-Year Cohort of Norwegian Births'. Obstetrics and Gynaecology, 89, 543-548. Cardozo L (1993) 'Is routine induction of labour at term ever justified'? British Medical Journal, 306, 840-841. Cardozo L., Fysh J., Rearce J.M. (1986) 'Prolonged pregnancy: the management debate'. British Medical Journal, 293, 1059-1063. Caritis S.N., Thom E., McNellis, D. (1995) 'Comment on the effectiveness of induction of labor for post term pregnancy'. American Journal of Obstetrics and Gynaecology, 172 (1), 240-241. Crowley P. (2002) 'Interventions for preventing or improving the outcome of delivery at or beyond term' (Cochrane Review). In: The Cochrane Library, Issue 1, Oxford: Update Software.
Hilder L., Costeloe, K., Thilaganathan, B. (1998) 'Prolonged pregnancy: evaluating gestation-specific risks of fetal and infant mortality'. British Journal of Obstetrics and Gynaecology, 105, 169-173. Ingemarsson I, Kallen K. (1997) 'Stillbirth and the rate of neonatal deaths in 76,761 post term pregnancies in Sweden, 1982-1991:a register study'. Acta Obstetrics and Gynaecology Scandinavia, 76: 658-662. Keirse M.J.N.C. (1993) 'Post term Pregnancy: New Lessons from an Unresolved Debate'. Birth, 20 (2), 102-105. Martin D.H., (1978) 'A randomised controlled trial of selective planned delivery'. British Journal of Obstetrics and Gynaecology, 85, 109-113. Martin J.N., Sessums J.K., Howard P., Martin R.W., Morrison J.C. (1989) Alternative approaches to the management of gravidas with prolonged post term postdate pregnancies. Journal Miss State Medical Association, 30, 105-111.
Dublin S., Mona L.R., Kaplan R., Watts D., Heather M.D., Critchlow C. (2000) 'Maternal and neonatal outcomes after induction of labour without an identified indication'. American Journal of Obstetrics and Gynaecology, 183 (4), 986-994.
Menticoglou S.M., Hall P.F.(2002) 'Routine induction of labour at 41 weeks gestation: nonsensus consensus'. British Journal of Obstetrics and Gynaecology, 109, 485-491. Misra M., Vavre S. (1994) 'Labour induction with intracervical prostaglandin E2 gel and intravenous oxytocin in women with a very unfavourable cervix'. Australian and New Zealand Journal of Obstetrics and Gynaecology, 34 (5) 511-515.
Duff C., Sinclair M. (2000) 'Exploring the risks associated with induction of labour: a retrospective study using the NIMATS database'. Journal of Advanced Nursing, 31 (2), 410--417.
Neilson J.P. (2003) 'Routine ultrasound in early pregnancy' (Cochrane review) In: The Cochrane Library, Issue 4, Chichester, UK: John Wiley and Sons Ltd.
Cunningham F., Gant N., Leveno K., Gilstrap L., Hauth J., Wenstrom K. (1997) Williams Osbtetrics (21st ed.) New York: McGraw-Hill.
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Olesen A.W., Westergaard J.G., Olsen M.S. (2003) 'Perinatal and maternal complications related to post term delivery: A national register-based study, 1978-1993'. American Journal of Obstetrics and Gynecology, 189, 222-227. Sanchez-Ramos L., Olivier, F., Delke I., Kaunitz A.M. (2003) 'Labour induction versus expectant management for post term pregnancies: a systematic review with meta-analysis'. Obstetrics and Gynaecology. 101 (6) 1312-1318. Sims M.E., Walther F.J. (1989) 'Neonatal Morbidity and Mortality and Long-term Outcome of Postdate Infants'. Clinical Obstetrics and Gynaecology, 32 (2) 285-293. The Consultative Council on Obstetric and Paediatric Mortality and Morbidity (2003) 'Annual Report for the Year 2001: Incorporating the 40th Survey of Perinatal Deaths in Victoria'. The Consultative Council on Obstetric and Paediatric Mortality and Morbidity, Melbourne 2003(online). Available: http://www, health.vic.gov.au/perinatal/downloads/ccopm m_ann rep_ 2001 .pdf The National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units (1994) 'A clinical trial of induction of labour versus expectant management in post term pregnancy'. American Journal of Obstetrics and Gynaecology. 170, 716-723. World Health Organisation (1996) 'Safe Motherhood: Care in Normal Birth: a practical guide'. World Health Organisation, Geneva. Yogev Y., Ben-Haroush A., Gilboa Y., Chen R., Kaplan B., Hod M. (2003) 'Induction of labor with vaginal prostaglandin E2. Journal of Maternal-Fetal and Neonatal Medicine, 14 (1) 30-34.
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Australian Midwifery Journal 2005,18 (2)
The risks associated with post term pregnancy: a literature review Juliana Brennan RN, Grad Dip Midwifery, M Mid
Angliss Health Service Family Birth Centre, Ferntree Gully, Victoria, Australia Correspondence Juliana Brennan Email julianabrennan @optusnet.com.au
Abstract The purpose of this paper is to conduct a critical literature review of the risks associated with induction of labour and a conservative approach to post term pregnancy. The main aim was to establish whether a conservative approach to post term pregnancy is associated with increased rates of perinatal mortality and morbidity, and whether induction of labour reduces these rates. Electronic databases and texts were examined. The findings were that the rates of caesarean section, instrumental birth, use of analgesia, incidence of fetal heart rate abnormality, meconium aspiration syndrome and fetal size were similar in both approaches to care. It appears that perinatal mortality rates increase in post term pregnancy yet the literature varies as to when this increase becomes significant. Induction of labour after 41 weeks gestation reduces the rates of perinatal mortality, however, the amount to which mortality rates are decreased by performing induction of labour at this gestation also varies within the literature. Therefore, it is difficult to give concise dates about when induction of labour should be recommended. Women should be informed of the risks associated with both approaches to care, and based on the review findings, they should be offered induction of labour between 291 days and 294 days, or between 41+4 and 41+7 weeks gestation. However, their preference for either approach should be respected.
Introduction This paper presents a critical review of the research literature on the risks associated with post term pregnancy. The purposes of this paper are first, to establish whether perinatal mortality and morbidity rates increase in pregnancy advancing beyond term, and whether induction of labour decreases the rates of perinatal mortality. Second, the paper seeks to utilise the information gathered from the critical review to develop recommendations for midwives to use when informing women about the risks associated with post term pregnancy. Background The definition of post term pregnancy is contested by some within the literature who argue that post term pregnancy is pregnancy of greater than 287 days, or 41 completed weeks of gestation (Halliday & Riley, 2003; Goeree et al., 1995; The National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units (NICH) et al., 1994; Keirse, 1993). The internationally recognised definition of post term pregnancy, however, is 42 completed weeks, or 294 days or more from the first day of the last menstrual period (Cunningham et al., 1997). Many research studies and texts also support this definition (Calder et al., 2001; Dublin, et al., 2000; Enkin et aL, 2000; World Health Organisation, 1996; Alfirevic & Walkinshaw, 1995; Sims & Walther, 1989; Augensen et al., 1987). The variations in definitions has led to different timings of when labour is induced in the induction arms of the Randomised Controlled Trials (RCT) comparing a conservative approach and induction of labour. This has made the interpretation of the optimum duration of pregnancy difficult (Gulmezoglu & Crowther, 2004). Furthermore, the various terms used to describe gestation are often unclear and confusing. The most recent Cochrane review assessing the effects of interventions for preventing or 10
improving the outcome of delivery at or beyond term states that routine induction of labour "after 41 weeks gestation" appears to reduce perinatal mortality rates (Crowley, 2002, p.1). The term "after 41 weeks" is ambiguous and vague. Does this mean that any time from 41 completed weeks gestation is an acceptable time to recommend induction of labour? It can only be assumed that this means any time after 41 completed weeks gestation, or after 287 days, and for clarity purposes, it would have been less ambiguous to have used the term "from 41 completed weeks gestation", or "from 287 days". It appears that perinatal mortality rates increase with post term pregnancy, however, there are conflicting views as to when these increases become significant. A retrospective study conducted by Hilder et al., (1998), shows that when stillbirth, neonatal, and post neonatal (infant losses up to one year of life) mortality rates are combined, they increase significantly in term and post term pregnancy (see Table 1 and 2). A cross-sectional study by Olesen et aL, (2003) has found that perinatal mortality rates significantly increase from 42 completed weeks gestation, or 294 days. However, some individual RCTs have found no significant differences in perinatal mortality rates after 41 completed weeks gestation, or from 287 days (Hannah et al., 1996; The National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units (NICH) et al., 1994; Cardozo, 1993; Hannah et al., 1992). A quality cohort study has found no significant increase in perinatal mortality rates even after 42 completed weeks, or 294 days (Campbell et al., 1997). A Cochrane systematic review provides evidence that 'routine' induction of labour after 41 weeks gestation reduces the risk of perinatal death in normally formed babies (OR 0.23; 95% CI 0.06 to 0.90) (Crowley, 2002). However, another systematic review performed by Sanchez-Ramos et al., (2003) has found that induction of labour at or just after 41 weeks gestation does not significantly Australian Midwifery Journal 2005, 18 (2)
Juliana Brennan
reduce perinatal mortality rates (OR 0.41; 95% CI 0.14 to 1.18). It has also been suggested that induction of labour may be associated with an increase in perinatal morbidity, especially when the cervix is not favourable for induction (Gulmezoglu & Crowther, 2004; Duff et al, 2000). However, a conservative approach has also been identified as being associated with an increase in perinatal morbidity (Gulmezoglu & Crowther, 2004; Olesen et al., 2003; Sanchez-Ramos, et al., 2003; Campbell et aL, 1997). These variations in findings have led to much controversy and confusion over how women with post term pregnancy should be cared for. Therefore, conducting a critical review of the literature was important. It has allowed for the clarification of definitions and terms used to describe post term pregnancy. Importantly, practice recommendations have been developed providing midwives with information to share with women based on the best available evidence. The reported frequency of pregnancy extending beyond 42 completed weeks gestation (294 days) varies from 3.5% to 14% (Menticoglou & Hall, 2002; Enkin et al., 2000; Hilder et aL, 1998; Sims & Walther, 1989; Cardozo et al., 1986). In the State of Victoria, pregnancy advancing beyond 41 weeks gestation has steadily declined from 4% in 1986 to 1.2% in 2002. This may be due to more accurate pregnancy dating through the use of early pregnancy ultrasound (Enkin et al., 2000), however, the increasing rates of inductions of labour are probably the most likely cause of this decline. Post term pregnancy, defined by Halliday and Riley (2003) as pregnancy advancing beyond 41 completed weeks (287 days) gestation, was the single most common reason for inducing labour in 2002 in the State of Victoria, and 24.2% of all inductions were performed as a result of pregnancy extending beyond this gestation. It has been suggested that the most frequent cause in apparent post term pregnancy is an error in pregnancy dating (Crowley, 2002; Neilson, 2003). These errors may be reduced by the use of early ultrasound in pregnancy (Neilson, 2003). Although the cause of truly post term pregnancy is unknown (Alfirevic & Walkinshaw, 1994), there are identifiable risk factors for post term pregnancy. Primiparity is the most common risk factor (Campbell et al., 1997; Alfirevic & Walkinshaw, 1994), and there may be a genetic predisposition for some women to have more prolonged pregnancies than others (Olesen et al., 2003; Campbell et aL, 1997) Fetal chromosomal abnormalities are also associated with post term pregnancies (Hilder et al., 1998; Campbell et al., 1997).
Literature search strategy A comprehensive literature review was performed using text books, Medline, Pub Med, Cinahl, MIDIRS and the Cochrane Database. Search headings included; ! Risks associated with induction of labour versus a conservative approach to post term pregnancy I Maternal morbidity associated with induction versus a conservative approach which included instrumental vaginal birth, perineal trauma, caesarean section rates, narcotic analgesia, and epidural anaesthesia. i Neonatal morbidity associated with induction versus a conservative approach which included meconium liquor, meconium aspiration, neonatal seizures, asphyxia, birth injuries, low 5 minute apgar scores, incidence of intubation, and incidence of fetal heart rate abnormality. ! Perinatal mortality. Australian Midwifery Journal 2005, 18 (2)
Overall, a range of Systematic reviews, RCTs, Cohorts, retrospective, observational, qualitative studies, reviews and commentaxies were obtained. All were of varying quality, date ranges between 1969 and 2004. There have been very few published RCTs since 1992 when Hannah et al., (1992) conducted a RCT which methodologically is considered a high quality study (Grant, 1994). The articles were mainly in the medical literature, with limited research conducted by midwives. Articles published in languages other than English were excluded from review.
Risks associated with induction of labour Findings in relation to risks associated with induction of labour compared with a conservative approach for the care of women with post term pregnancy vary within the literature. Results from a retrospective study suggests that induction of labour has certain risks to both mother and baby when compared with a conservative approach to post term pregnancy. Some of the reported risks to the mother of induction of labour include increased rates of mean average blood loss, caesarean section and increased use of epidural during labour (Duff et al., 2000). Risks to the fetus and neonate include the possibility of fetal distress, bruising, and increased incidence of lower apgar scores (Duff et al., 2000). Some individual RCTs also found increased risk associated with induction of labour compared with a conservative approach to post term pregnancy (Augensen, et al. 1987; Cardozo et al., 1986). Induction of labour has been found to be associated with an increase in perineal trauma, fetal distress, lower apgar scores, higher rates of intubation of neonates (Cardozo, 1986), and an increase in the incidence of neonatal jaundice (Augensen et aL, 1987). However, a Cochrane systematic review conducted by Crowley, (2002) evaluated the effect of induction of labour on various maternal and neonatal morbidities in post term pregnancy. The review found that induction of labour after 41 weeks gestation had no statistically significant effect on instrumental birth rate, use of analgesia, or incidence of fetal heart rate abnormality when compared with a conservative approach to post term pregnancy. These findings, however, may not be valid because all the RCTs used in the systematic review were subject to the possibility of detection bias, as blinding as to mode of onset of labour was impossible (Crowley, 2002). Therefore, firm conclusions cannot be made regarding instrumental birth rate findings (Crowley, 2002). Nevertheless, this is the best available evidence on the effect of induction of labour on maternal and neonatal morbidity in post term pregnancy to date. There are widely held beliefs that induction of labour for post term pregnancy is associated with higher caesarean section rates (Duff et al., 2000; Enkin, 2000). The Canadian Multi-Centre Post term Pregnancy Trial conducted by Hannah et aL, (1992) claims that induction of labour beyond 41 weeks gestation (from 287 days) was, in fact, more likely to reduce the rate of caesarean section than a conservative approach to post term pregnancy. This finding was also made in a systematic review by Sanchez-Ramos et aL, (2003). However, the validity of the findings of the Canadian Multi-Centre Post term Pregnancy Trial in relation to the caesarean section rates have been criticised (Crowley, 2002; Mentacoglou & Hall, 2002; Keirse, 1993). One major problem with the RCT conducted by Hannah (1992) according to Keirse (1993) is that endocervical administration of prostaglandin (PGE2) was routinely available to all women assigned to the induction group, but was not part of the policy if induction was performed in the conservative approach group. According to Keirse (1993), 77.2% of the induction group 11
Juliana Brennan
Table 1 Prospective risk of stillbirth (SB), neonatal (NN) and post neonatal (PNN) death and pregnancy loss rates per 1000 ongoing pregnancies (% op) at each week of gestation (statistics from Hilder et aL (2000) and Hilder et aL (1998)
Gestation (weeks) 35 36 37 38 39 40 41 42 43
No of OP
No of SB
161 638 159 723 155 791 147 631 126 448 93 539 39 245 10 305 1 874
48 62 47 77 62 81 50 16 4
Risk of SB/1000 OP (95% Cl) 0.30 0.39 0.30 0.52 0.49 0.87 1.27 1.55 2.13
Risk of SB in ensuing week
(0.23-0.37) (0.31-0.46) (0.23-0.37) (0.44-0.60) (0.40--0.58) (0.80-0.96) (0.94-1.60) (0.93-2.78) (0.28-3.99)
received PGE2, compared with only 27.7% of those women from the conservative approach group who were induced. It has been reported in some literature that the incidence of operative delivery overall is lower in women induced with prostglandin (Enkin et al., 2000). Therefore, there is likely to be a difference in caesarean section rate in the study by Hannah et al., (1992), as the conservative management group were not offered the best treatment option (Keirse, 1993). In argument against this finding, the Consultative Council on Obstetric and Paediatric Mortality and Morbidity (2003) states that the use of PGE2 has been associated with an increased rate of fetal death, and recommends exercising care when using prostglandins for women with an unfavourable cervix. Yet no research evidence has been found to support this claim. Yogev et al., (2003) states that no serious complications occur with the use of PGE2 in women with post term pregnancy (of more than 294 days) and Misra and Vavre (1994) state that perinatal mortality is not affected when comparing the use of PGE2 and oxytocin in women with a very unfavourable cervix. The Cochrane Review states that routine induction of labour for post term pregnancy is not associated with an increased risk of caesarean section, regardless of parity, state of the cervix, or method of induction. The rates of caesarean section are similar among both approaches to care (Crowley, 2002). C o n s e r v a t i v e a p p r o a c h a n d perinatal m o r b i d i t y
Most of the literature confirms that there is an increase in the incidence of meconium stained amniotic fluid in pregnancies extending beyond 41 weeks (from 287 days), (Enkin et al., 2000; Crowley, 2002; Grant, 1994; Augensen et al., 1987). However, although there is an increased incidence of meconium stained amniotic fluid, it is well documented that induction of labour does not reduce the risk of meconium aspiration syndrome. (SanchezRamos et al., 2003; Crowley, 2002; Enkin et al., 2000; Grant, 1994) or neonatal seizures (Crowley 2002; Enkin et al., 2000). Therefore, it is questionable whether induction of labour performed for this reason is beneficial. A cross sectional study by Olesen et al., (2003) has found there to be significantly increased risks of maternal complications such as post partum haemorrhage, cephalopelvic disproportion, cervical rupture, shoulder dystocia and puerperal infection when pregnancy extends beyond 42 completed weeks gestation, or 294 days (Olesen et al., 2003). When the risk of maternal and fetal complications were stratified on birthweight, it was found that there were more maternal complications with birthweights < 2500 g and with birthweights > 4500 g than in birthweights of 2500--4499g (Olesen et al., 2003). 12
1:3332 1:2536 1:3332 1:1922 1:2039 1:1148 1:786 1:644 1:486
Stillbirth rate (% OP) 0.33 0.45 0.35 0.56 0.57 0.86 1.27 1.55 2.12
NN & PNN Mortality Rate (% OP)
Total pregnancy loss rate (% OP)
0.16 0.38 0.34 0.70 0.83 1.57 1.48 3.29 3.71
0.5 0.8 0.7 1.3 1.4 2.4 2.8 4.8 5.8
A Cohort study conducted by Campbell et al., (1997) also states that maternal complications were generally associated with larger fetal size (LGA) (defined as birthweight equal or greater than the 90th percentile) (Campbell et al., 1997). Labour dysfunction and obstetric trauma were associated with both LGA and post term birth, however shoulder dystocia was associated with LGA only. The RCTs to date do not appear to have examined matemal complications of post term pregnancy, other than caesarean section, instrumental birth rates and use of analgesia. However, there are limited numbers of RCTs that have examined fetal size between induction of labour and conservative approach groups. These have stated that no differences in birthweights were found between groups (NICH et al., 1994; Egarter, 1989; Cardozo, 1986). Therefore, the best available evidence suggests that induction of labour does not have an effect on fetal size when compared with a conservative approach to post term pregnancy. Perinatal m o r t a l i t y rates a n d post t e r m p r e g n a n c y
It is stated in the literature that there are increased risks of perinatal mortality as pregnancy advances beyond 42 weeks gestation, or 294 days (Gulzemoglu & Crowther, 2004; Olesen et al., 2003). However, there is also literature to support that perinatal mortality rates increase prior to this gestation (Hilder et aL, 1998). In a retrospective analysis of 171,527 births in the North East Thames region of London between 1989-1991, it has been found that the rate of stillbirth, expressed as ongoing pregnancies, increases slightly in each ensuing week at and beyond 280 days (see Table 1 & Table 2). Similar rates of increases can be seen in Denmark, Sweden, New Zealand, New York City and England (Olesen et al., 2003; Menticoglou & Hall, 2002; Hilder et al., 1998; Ingemarsson & Kallen, 1997). However, when stillbirth, neonatal and post neonatal mortality rates (infant losses up to one year of age) are examined, there is a significant increase in mortality rates in term and post term pregnancy (see Table 1 & Table 2) (Hilder et al., 1998; Hilder et al., 2000). It is interesting to note that although some of these infant deaths result from perinatal complications, most have no known cause (American College of Obstetricians and Gynaecologists, 2004). One explanation offered by Hilder et al. (1998) as to why there is a significant increase in mortalities among these infants, is that post term pregnancies may well be compromised before birth. If this is the case, then induction of labour may not decrease the rate of infant losses up to one year of age. Induction of labour at an earlier gestation may simply shift these losses from post term births, to term births. One limitation to the retrospective analysis performed by Australian MidwiferyJournal 2005, 18 (2'.
The risks associated with post term pregnancy: a literature review
Table 2 Rate of stillbirth compared with infant losses up to 1 year by gestational age (Statistics adapted from Hilder, et aL (1998) Rate of stillbirth compared with infant losses up to 1 yr by gestational age. Adapted from Hilder et aL, (1998) Total infant losses (up to 1 yr)
._~~'~ 0~ r ~ .go O3r
stillbirths
6 Q" ~
5
t - t~ ..~ O~ t~
4
._ ~
o-~_o ~o-~ ~ c w ~ rr
O e-
9 0 ~ o.o..4
2
0 35
36
37
38
39
40
41
42
43
W e e k s of gestation
Hilder et al., (1998) is that pregnancies with chromosomal abnormalities were not eliminated from the study. It is widely acknowledged that pregnancies affected with chromosomal abnormalities are associated with post term pregnancy (Hilder et aL, 1998; Campbell, 11997). Therefore, this is likely to affect the mortality rates presented in this study. A cross sectional study by Olesen et aL (2003) has found that the risk of perinatal death was significantly higher in pregnancy advancing beyond 42 completed weeks gestation, or 294 days. However, this study acknowledges that the results give no indication as to whether this risk may be reduced by inducing labour before 42 weeks of gestation (before 294 days). A 10-year cohort study was conducted in Norway assessing risk factors and birth outcomes for post term pregnancy (Campbell et al., 1997). The results from this study show that there was only a slightly increased risk of perinatal mortality in post term (294 days or more) as compared with term births. This study included stillbirths and early neonatal losses, of less than 168 hours after birth. According to Campbell et al., (1997), the evidence for an adverse impact on perinatal mortality in post term birth is weak once other factors are taken into account. For post term births (from 294 days), the risk factors were small for gestational age (SGA), defined as birth weight less than the 10th percentile of all births of the same gestational age, and maternal age > 35 years. Large for gestational age (LGA) was a protective factor against perinatal mortality (Campbell et al., 1997). This has important implications for midwifery practice. Several studies have shown quite good sensitivity and specificity of fundal height measurements for predicting low birth weight for gestation (Enkin et al., 2000). Where low birth weight for gestation is suspected, midwives must be aware that this condition may constitute a 'high risk' situation. A Cochrane systematic review performed by Crowley (2002) examined 19 RCTs which showed one fetal death occurring in the induction group, compared with nine fetal deaths in the conservative group (total number of participants 7925). This difference is both clinically important and statistically significant (Enkin et aL, 2000). However, it appears that some of the nine deaths that occurred in the conservative management groups in the RCTs evaluated in the Cochrane review, may have occurred for reasons other than post term pregnancy. Caritis et al., (1995) states that one death occurring in the conservative management group (Cardozo, 1986), was as the result of Australian Midwifery Journal 2005, 18 (2)
acute abruptio placenta which may be a condition unrelated to post term pregnancy. There was also one fetal death due to a cord complication which could have happened at any time of gestation, and according to the author, could not be attributed to post term pregnancy (Egarter et al., 1989). Menticoglou and Hall (2002) adds to the critical review by saying that one further stillbirth occurred in a mother with an abnormal glucose tolerance test (GTT) (in Henry 1969). This stillbirth should have been excluded from the study, as gestational diabetes is not considered to be without increased risk, and this stillbirth may therefore have been due to a cause unrelated to post term pregnancy. Gestational diabetes was a criteria for exclusion from the Hannah (1992) trial. One further death occurred at 42 weeks in a 2600 g infant ascribed to fetal distress which presumably could have occurred and resulted in similar management difficulties during an earlier induction. A 2600 g weight at 42 weeks is a low birth weight, and the hypothesis that this death occurred directly as a result of gestation alone is questionable (Menticoglou & Hall 2002). Campbell et aL (1997) state that poor fetal growth is, indeed, the major risk factor for fetal death, regardless of gestation.
The effect of Induction of labour on perinatal mortality rates There have been numerous research studies comparing outcomes of induction of labour at various gestations with a conservative approach to post term pregnancy. The largest and methodologically most outstanding RCT conducted to date is the Canadian Multi-centre Post term Pregnancy Trial by Hannah et al. (1992). Although this trial has been criticised in relation to the claim that induction of labour reduces the rate of caesarean sections, this trial is not only the largest RCT, but methodologically it is the best of all the RCTs performed to date (Grant, 1994). This trial found that the rates of perinatal mortality and neonatal morbidity were similar with either inducing labour or adopting conservative approaches to the care of women with pregnancy extending beyond 41 weeks gestation (from 287 days). Although a higher rate of perinatal deaths occurred in the conservative approach group, the results were not statistically significant (Hannah et al., 1992). A systematic review conducted by Sanchez-Ramos et al. (2003) reviewed 16 RCTs and compared the outcomes of routine induction of labour and expectant management for uncomplicated pregnancies at and just after 41 weeks gestation. This review included 6588 subjects from 10 countries over 33 years. The review paid particular attention to homogeneity and only included RCTs with uncomplicated pregnancies. Sensitivity analysis revealed that no studies had a disproportionate impact on the outcomes. The findings were that a strategy of routine induction at or just after 41 weeks does not significantly decrease rates of perinatal mortality (OR 0.41; 95%CI 0.14, 1.18). The major weakness of this systematic review was identified by the authors as being a lack of an appropriate comparison group to elective induction at 41 weeks. In four RCTs, the physicians intervened at 43 weeks, in four others at 44 weeks, and two trials did not specify (Sanchez-Ramos et al., 2003). These issues can also be identified as being a limitation to the Cochrane review, as this review, along with the review by Sanchez-Ramos et aL (2003) examined some of the same RCTs. The most recent systematic review performed for Cochrane, comparing a policy of induction at and from > 40 weeks gestation 13
Juliana Brennan
(from 280 days) with a conservative approach, suggest a reduction in perinatal mortality rates with induction of labour after 41 weeks gestation (0.23 95% CI 0.06 to 0.90) (Crowley, 2002). It is interesting to note that some RCTs examined the effect of induction of labour at 40 weeks gestation, or at 280 days. In these trials, there was found to be a significant increase in instrumental vaginal births without any decrease in perinatal mortality rates, therefore induction of labour between 280 days and 286 days is unwarranted (Enkin et al., 2000; Grant, 1994). From the Cochrane systematic review, it has been suggested that approximately 500 inductions would have to be performed after 41 weeks to prevent one perinatal death from occurring (Crowley, 2002). Although, Crowley (2002) also states that the number of inductions required to prevent one perinatal death may be higher in present day practice as these figures may be biased by the high perinatal mortality rate reported in one of the trials used in the systematic review, which was a poor quality study by Hertry (1969). Unfortunately, no systematic reviews have examined the effect of induction of labour on reducing perinatal mortality rates when induction of labour is performed at 42 completed weeks gestation (294 days). Discussion
Cochrane systematic reviews are considered 'gold standard' from which to guide decisions for research and practice (Gulmezoglu & Villar, 2003). However, the methodological quality varies considerably among the 19 RCTs used in the Cochrane Systematic review (Crowley, 2002). In three of the trials used in the systematic review, there are worrying disparities between the numbers in the two arras of the trials (Crowley, 2002). One of the RCTs included in the systematic review (by Martin 1989) only included a total sample size of 22 women (Grant, 1994). Post-randomisation exclusions that cannot be reinstated present a problem in the Martin (1978) trial (Crowley, 2002). Although these problems were identified, these RCTs were not removed from the review. If the Henry (1969), Martin (1978), and Martin (1989) trials were eliminated from the systematic review, the perinatal mortality rates would be reduced by three deaths. This would mean that six deaths would have occurred in the conservative approach group versus one death in the induction of labour group. Based on these figures, approximately 600 inductions would have to be performed after 41 weeks (at and from 287 days) to prevent one perinatal death. Furthermore, if these RCTs, along with the deaths caused by placental abruption (Cardozo, 1986), cord complications (Egarter, 1989), and SGA neonate (Hannah, et al. 1992) were removed from overview, the perinatal mortality rates would be reduced by six deaths. This would mean that three deaths would have occurred in the conservative approach group versus one death in the induction of labour group. Based on these figures, > 1000 inductions would have to be performed after 41 weeks (from 287 days) to prevent one perinatal death. It appears from the literature that perinatal mortality rates increase in post term pregnancy, yet the results vary as to when these findings become significant. The findings also vary as to whether induction of labour significantly reduces perinatal mortality rates if performed at or from 41 weeks gestation. Therefore, it is difficult to give concise dates about when induction of labour should be recommended. However, based on the above information, it would be acceptable to offer women induction of labour from 291 days to 294 days of pregnancy (41+4-41+7). 14
For those women who have no other risk factors, and who wish to adopt a conservative approach to post term pregnancy, they must be informed that perinatal mortality rates appear to increase in pregnancy advancing beyond term, and that induction of labour performed after 41 weeks appears to reduce perinatal mortality rates (Crowley, 2002). It must also be mentioned to women that there has been a considerable amount of research performed in relation to the effectiveness of various methods of fetal surveillance in reducing perinatal mortality and morbidity in post term pregnancy, where a conservative approach rather than induction of labour is adopted. Unfortunately, there is little evidence from RCTs and other studies to suggest that any form of fetal surveillance is reliable in detecting post term pregnancy becoming 'high risk' (Crowley 2002; Alferevic & Walkinshaw 1995; Grant 1994). Nevertheless, women should be informed of the risks associated with both approaches to care, and their preference for either approach should be respected (Hannah et al. 1996). Practice recommendations
Based on the available evidence, women should be informed of the following; 1 The research has shown contradictory findings in relation to the role of induction of labour reducing perinatal mortality rates. The best available evidence suggests that induction of labour some time after, yet not at 41 weeks gestation (287 days), reduces perinatal mortality rates. Therefore, induction of labour should be offered to women between 291 and 294 days (41+4 and 41+7). 2 The review found that induction of labour after 41 weeks gestation had no statistically significant effect on instrumental birth rates, use of analgesia, or incidence of fetal heart rate abnormality when compared with a conservative approach to post term pregnancy. Induction of labour does not appear to have an effect on meconium aspiration syndrome or fetal size. Induction of labour between 280 days and 286 days is associated with significantly higher rates of instrumental vaginal births, without significantly decreasing perinatal mortality rates, and is, therefore, unjustified. 3 Women should be informed that the best available evidence suggests that approximately 500 inductions would have to be performed after 41 weeks to prevent one perinatal death from occurring compared with a conservative approach to post term pregnancy. This figure is likely to be greater than this in present day practice. 4 SGA fetuses, regardless of gestation, and fetuses of women older than 35 years of age are at greater risk of perinatal death. 5 Minimal evidence exists from RCTs and other studies to suggest that any form of fetal surveillance is reliable in detecting post term pregnancy becoming 'high risk'. Conclusion
The findings of this review are, that perinatal mortality rates appear to increase in advancing pregnancy beyond term, and that induction of labour after 41 weeks gestation reduces perinatal mortality rates. Induction of labour for post term pregnancy is not associated with adverse outcomes when compared with a conservative approach to post term pregnancy, and based on these findings, induction of labour should be offered to women between 291 and 294 days gestation (41 + 4 - 41+7). Further research is required into the causes of post term pregnancy, and better methods of identifying the post term fetus at risk of perinatal death. Australian MidwiferyJournal 2005, 18 (2)
The risks associatedwith post term pregnancy:a literature review
The main limitations to this review are that the methodological quality, the definitions of what constitutes post term pregnancy, and the timing for inductions of labour, all varied considerably in the RCTs used in the Cochrane Review by Crowley (2002), possibly making the conclusions difficult to interpret. It is possible that had only high quality, properly conducted RCTs be included in the Cochrane review, that perinatal mortality rates would not be significantly reduced by induction of labour after 41 weeks gestation, and that perinatal mortality rates would only be significantly reduced at a later gestation. Gulmezoglu and Crowther (2004) are conducting a new review for Cochrane, taking into account the limitations of the existing Cochrane review by Crowley (2002). Their hypothesis is that a policy of labour induction at or beyond term compared with a policy of awaiting spontaneous labour indefinitely, improves pregnancy outcomes for the infant and the mother.
Acknowledgement I would like to gratefully acknowledge the assistance provided to me by Kathleen Fahy and Louise Noorbergen. Thank you to my husband Tim, for your support.
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