Increase in prostaglandin bioavailability precedes the onset of human parturition

Prostaglandins, Leukotrienes and Essential Fatty Acids (1996) 54(3), 187-191 © PearsonProfessionalLtd 1996

Increase in prostaglandin bioavailability precedes the onset of human parturition Roberto Romero 1, Hernan Munoz 2, Ricardo Gomez 1, Mauro Parra 2, Max Polanco 2, Victor Valverde 2, Jorge Hasbun 2, Jaime Garrido 2, Fabio Ghezzi 1, Moshe Mazor 1, Jorge E. Tolosa 1, Murray D. Mitchell 3 1Perinatology Research Branch, Intramural Division/National Institutes of Child Health and Human Development (NICHD), Bethesda, MD, USA. 2University of Chile, Santiago, Chile. 3University of Utah, Salt Lake City, UT, USA.

Summary The traditional paradigm that prostaglandins (PGs) are of central importance in the initiation of labor has been challenged. A group of investigators has recently reported that the amniotic fluid concentrations of PGE2 and PGF2~ increase only late in the course of labor implying that 'the accumulation of prostaglandins in amniotic fluid is an after-effect of labor and not indicative of a role of these compounds in the initiation of human parturition'. The present study was conducted to determine whether amniotic fluid prostaglandin concentrations increase prior to the onset of human labor, the central question in this controversy. Three amniocenteses were performed in 17 women with intrahepatic cholestasis of pregnancy - the first two prior to the onset of labor and the third during early spontaneous labor. PGE~ and PGF2~ were measured with sensitive and specific radioimmunoassays. Amniotic fluid concentrations of PGE 2 and PGF2~ increased prior to the onset of spontaneous labor. An additional increase in the concentrations of PGE2 and PGF2~ was found in samples obtained in early labor. We conclude that an increase in prostaglandin bioavailability precedes the onset of spontaneous human parturition.

INTRODUCTION

In the last two decades, prostaglandins (PGs) have been considered the central mediators of the onset of labor. 1-4 Recently, this view has been challenged by a group of investigators who proposed that prostaglandins are released as a consequence of labor and are not the cause of labor, s,~ This tenet is based upon the observation that the amniotic fluid concentrations of PGE 2 and PGF2~ did not increase until labor was established and cervical dilatation was greater than 2.5 cm. s,6 These data are contradicted by those recently reported by our group demonstrating that prostaglandin concentrations in amniotic

Received 8 May 1995 Accepted 16 June 1995 Correspondence to: Roberto Romero, Tel. 202 687 2329, 202 687 2537; Fax. 202 687 2607

fluid are increased in early labor. 7,8 Of course, the crucial issue is whether prostaglandin bioavailability increases prior to, rather than after, the onset of labor. The current study was undertaken to determine if amniotic fluid concentrations of PGE2 and PGF2~ increase prior to the initiation of human labor. PATIENTS AND M E T H O D S

Transabdominal amniocenteses were performed at weekly intervals after term had been reached, starting approximately at the 37th week of gestation in patients with intrahepatic cholestasis of pregnancy. The diagnosis of intrahepatic cholestasis of pregnancy was made when a patient met the following criteria: (i) continuous and generalized pruritus appearing during the last trimester of pregnancy and resolving after delivery; (ii) a negative history of any medical disorder associated with pruritus 187

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and absence of drug intake other than iron and vitamins; and (iii) absence of fever and generalized malaise. Patients with the icteric form of the disorder were excluded because they underwent induction of labor as soon as fetal lung maturity was established. The clinical indication for serial amniocenteses was to determine the presence of meconium-stained amniotic fluid. This procedure has been considered the most useful method for fetal evaluation in patients with intrahepatic cholestasis of pregnancy31° Meconium-stained fluid has been reported in up to 31% of cases and is associated with a higher rate of perinatal mortality, n In two large studies of intrahepatic cholestasis of pregnancy encompassing 20 years, all perinatal deaths occurred in pregnancies with meconium-stained amniotic f l u i d . 11'12 The finding of meconium was used as an indication for induction of labor. If induction was contraindicated, an elective cesarean section was performed. This study was approved by the Human Investigations Committee of the University of Chile. All patients gave oral consent to allow the use of fluid which remained after clinical tests for research purposes. 17 patients gave written consent to undergo an additional transabdominal amniocentesis under ultrasound guidance while in early labor (cervical dilatation < 4 cm). This procedure was performed for research purposes. No financial compensation was given to patients for participating in this study. Of 106 patients with intrahepatic cholestasis of pregnancy who agreed to participate in the study, 28 patients had two amniocenteses before the onset of labor and among these 17 patients had an additional experimental amniocentesis in early labor. All patients with meconiumstained amniotic fluid were excluded because of the unknown effect of meconium on prostaglandin concentrations in amniotic fluid. All patients entered labor spontaneously, had a normal partogram, and had a spontaneous vaginal delivery of a healthy term newborn. Amniotic fluid concentrations of PGE2 and PGF2~ were determined with a sensitive and specific radioimmunoassay validated for use with h u m a n amniotic fluid. These methods have been previously described in detail by our g r o u p . 11,I2 Briefly, aliquots of amniotic fluid were acidified to pH 3 with citric acid (3.7 mol/1) and 5 ml of anhydrous diethyl ether was added to 1 ml of fluid (J.T Baker Inc, Philipsburg, NJ). After centrffugation, the upper organic layer was removed and evaporated to dryness under a stream of nitrogen. The residue was dissolved in 1 ml of assay buffer. Radioimmunoassays were performed with antisera from Advanced Magnetic Inc (Cambridge, MA). The recovery of PGE2 and PGF2~ from the extraction procedure was 9 9 % and 9 0 0 respectively. Interassay and intraassay coefficients of variation for PGE2 were 5.9% and 8.5% and for PGF2~ were 6.2% and 9.5%, respectively. Statistical analysis was performed with ANOVA for re-

peated measures and Duncan test for post-hoc analysis for patients with three amniotic fluid samples. A paired Student t-test was used to compare prostaglandin concentrations of the 28 patients with two determinations. Logarithmic transformation of prostaglandin concentration was conducted prior to inferential statistical analysis. A P value < 0.05 was considered significant. RESULTS

Concentrations of PGE2 and PGF2~ were significantly higher in the amniotic fluid retrieved from the second amniocentesis than in the fluid retrieved from the first amniocentesis; (P< 0.01 for each prostaglandin when 28 patients were considered and P < 0.05 when only the 17 patients with three amniocentesis were studied) (Figs 1 and 2). In addition, amniotic fluid concentrations of PGE2 and PGF2~ were higher in samples obtained in early labor than in samples obtained at the time of the second amniocentesis (not in labor). Table 1 displays the descriptive statistics of the 17 patients with three amniocenteses. DISCUSSION

A pivotal issue in the study of h u m a n parturition is whether prostaglandins mediate the onset of labor. The evidence traditionally invoked to support their role is based upon the capacity of these compounds to induce labor or abortion 1-4,13and that of prostaglandin synthase inhibitors to arrest preterm labor. TM However, these effects may simply represent a pharmacologic rather than a physiologic effect. Therefore, much of the evidence for the participation of prostaglandins in the initiation of labor at term is derived from studies which demonstrate an increase in prostaglandin concentrations in maternal plasma and amniotic fluid during labor. 15-19 Recently, the observations that prostaglandin concentrations increase in amniotic fluid during labor have been suggested to be invalid because fluid for these studies was collected transvaginally. 2° Prostaglandin concentrations in fluid obtained by this method may yield spurious results due to contamination of the sample with vaginal secretions, which contain prostaglandins. 6,21 Indeed, prostaglandin concentrations in fluid retrieved trasvaginally are significantly higher than those obtained transabdominally, ~,21and thus may not be representative of physiologic events in the entire amniotic cavity. The most serious challenge to the traditional role of prostaglandins as physiologic mediators for the onset of labor derives from the observation that when amniotic fluid is obtained transabdominally, PGE2 and PGF2~ concentrations are not increased in early labor ~ but only after cervical dilatation has reached 4 cm or more. These findings have been interpreted to indicate that pros-

Prostaglandins, Leukotrienes and Essential Fatty Acids (1996) 54(3), 187-191

© Pearson Professional Ltd 1996

Increase in prostaglandin bioavailabifity precedes the onset of human parturition

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Fig. 2 Serial amniotic fluid PGF2~ concentrations in 28 patients according to labor status (O 1st amniocentesis no labor; • 2rid amniocentesis - no labor; • 3rd amniocentesis - early labor).

taglandins are released into the amniotic cavity after the onset of labor. However, these data are contradicted by two studies in which amniotic fluid prostanoid concentrations were found to be increased during early labor in w o m e n at term with either intact or ruptured membranes/,8 However, this evidence must also be considered circumstantial. A causal relationship between prostaglandins and labor cannot be accepted unless it can be demonstrated that the concentrations of prostaglandins increase before the onset of labor. It has been argued that there is no evi© Pearson Professional Ltd 1996

dence that prostaglandin concentrations increase in any compartment before the onset of labor. The results of our study provide unequivocal evidence that an increase in PGE2 and PGF2~ occurs prior to the onset of parturition. A limitation of this study is that all patients had intrahepatic cholestasis of pregnancy. It is theoretically possible that labor in this condition may not be similar to that of normal women. However, the mean concentrations of PGE 2 and PGF2~ reported in this study are very similar to those previously reported by our group in a cross-sectional study of w o m e n without cholestasis

Prostaglandins, Leukotrienes and Essential FattyAcids (1996) 54(3), 187-191

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Roberto Romero et al

Table 1 Descriptive statistics for serial amniotic fluid PGE2 and PGF2,~concentrations in late pregnancy and early labor (n = 17).

Variables

Mean

SEM a

Median

Range

25th quantile

75th quantile

0.36

0.07

0.25

0.04-1.27

0.2

0.43

2rid amniocentesis no labor 3rd amniocentesis early labor

0.67

0.16

0.48

0.14-2.68

0.25

0.8

1.94

0.4

1.09

0.18-5.5

0.6

3.15

1 st amniocentesis no labor

0.42

0.11

0.30

0.08-1.82

0.55

0.48

2rid amniocentesis-

0.88

0.21

0.62

0.19-3.46

0.28

1.15

2

0.32

1.85

0.13-4.5

1.05

3.15

PGE2

1st amniocentesis -

(ng/ml)

no labor

PGF2~, (ng/ml)

no labor

3rd amniocentesis early labor SEM, standard error of the mean.

of p r e g n a n c y 2 2 It is e x t r e m e l y difficult to c o n d u c t a s t u d y w i t h serial a m n i o c e n t e s e s in n o r m a l w o m e n . By e x c l u d i n g patients with any other maternal and neonatal pathology a n d also t h o s e w i t h t h e icteric f o r m of c h o l e s t a s i s (severe), w e h a v e s o u g h t to m i n i m i z e t h e p o t e n t i a l for effects of pathologic conditions on our observations. In c o n c l u s i o n , o u r r e s u l t s i n d i c a t e t h a t a n i n c r e a s e in amniotic fluid prostaglandin concentrations occur prior to t h e o n s e t of h u m a n labor. T h e s e r e s u l t s are c o n s i s t e n t w i t h similar f i n d i n g s r e p o r t e d in s h e e p z2 a n d r h e s u s m o n k e y s . 23,24 Collectively, t h e s e o b s e r v a t i o n s p r o v i d e s t r o n g s u p p o r t for t h e r o l e of p r o s t a g l a n d i n s in t h e initiat i o n of labor.

7.

8.

9. 10.

11. ACKNOWLEDGEMENTS

Drs Hernan Munoz, Ricardo Gomez and Fabio Ghezzi are recipients of scholarship awards from the Fogarty Foundation, National Institutes of Child Health and H u m a n Development.

REFERENCES

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Prostaglandins, Leukotrienes and Essential Fatty Acids (1996) 54(3), 187-191