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Ultrasound assessment of cervical dynamics during the first stage of labor
European Journal of Obstetrics St Gynecology and Reproductive Biology 53 (1994) 123-127
Ultrasound assessment of cervical dynamics during the first stage of labor H.J.H.M. van Dessel*a, J.H.M. Frijnsb, F.Th.J.G.Th.
Kok”, H.C.S. Wallenburg”
‘Institute of Obstetrics and Gynecology, Erasmus University School of Medicine and Health Sciences, EE 2283, P. 0. Box 1738. 300 DR Rotterdam, The Netherlands bDepartment of Otolaryngology, University Hospital, Leiden. The Netherlancis ‘Department of Obstetrics and Gynecology, Westeinde Hospital, The Hague, The Netherlands
(Accepted 3 November 1993)
Abstract Objective: Assessment of cervical dynamics during the first stage of labor by a combination of ultrasound cervimetry and intrauterine tocography. Methods: Sixty-two parturients were divided into four groups: nulliparous women in spontaneous (n = 9) or oxytocin-induced labor (n = 26), parous women in spontaneous (n = 11) or oxytocin-induced labor (n = 16). Intrauterine pressure and cervical dilatation were continuously recorded and assessed by off-line computer analysis. Results: All women with spontaneous labor showed cervical responses to uterine contractions at the beginning of the recording. The first cervical response to a uterine contraction occurred at a significantly smaller dilatation in parous than in nulliparous women with induced labor (2.9 and 3.6 cm, respectively). Also, acceleration of cervical dilatation occurred at less dilatation in parous than in nulliparous women (3.4 cm and 4.8 cm, respectively), and myometrial work per cm of cervical dilatation was less in parous than in nulliparous parturients. Conclusions: The results indicate significant differences between cervical dilatation patterns in nulliparous and parous women, which may be due to structural cervical changes caused by labor and parturition. The labor patterns found were different from those originally described by Friedman (Friedman EA. Graphic analysis of labor. Am J Obstet Gynecol 1954; 68: 1568-1575), as no deceleration phases were detected. Ultrasound cervimetry is a valuable technique for the study of cervical dynamics during labor. Key words:
Ultrasound
cervimetry; Cervical dilatation;
First stage of labor
1. Introduction Although digital assessment of cervical dilatation is generally applied in clinical obstetrics, the intermittent nature of the procedure and its low accuracy and precision limit its usefulness for the study of cervical behavior during labor [ 11. In earlier reports we described a device for continuous measurement of cervical dilatation using ultrasound [2,3]. A similar device has been reported by others [4,5]. In combination with a tocograph, the ultrasound cervimeter provides continuous quantative data on cervical dynamics during labor. It consists of two small ultrasonic transducers attached to the lateral rims of the dilating cervix and connected to an electronic processing unit. Ultrasound bursts are transmitted by one transducer and received by the other one, and the dis-
tance between the transducers is calculated electronically from the transmission time. Using this technique we studied the dynamics of the human cervix during the first stage of spontaneous and oxytocin-induced labor with the aim of assessing cervical dilatation patterns in association with intrauterine pressure. 2. Material and methods 2.1. Patients Subjects were recruited from pregnant women under obstetric care in the antenatal clinic of the AZR-Dijkzigt Hospital. The women selected met all of the following criteria: (i)
* Corresponding
author.
0028-2243/94/$07.00 0 1994 Elsevier Science Ireland SSDI 0028-2243(93)01733-A
Ltd. All rights reserved.
uncomplicated pregnancy cephalic position;
with a single fetus in
124
H. J. H. M. van Dessel et al. /Eur. J. Obstet. Gynecol. Reprod. Biol. 53 (1994)
(ii) duration of pregnancy between 38 and 42 weeks; and (iii) admission to the labor room in the first stage of spontaneous labor or for scheduled elective induction of labor. Spontaneous labor was defined as the occurrence of regular painful contractions every 3-5 min in a woman with a cervix score according to Burnhill et al. of 6 or more [6]. Elective induction of labor was defined as induction of labor in the absence of a medical-obstetric indication for termination of pregnancy [7]. Elective induction is an accepted procedure in our Department, and was performed in the presence of a cervix score of 6 or more only. Sixty-seven term parturients gave their informed oral consent and were enrolled in the study. Of the 67 recordings, live were of insufficient quality, mainly due to early disconnection of the cervical transducers; 62 recordings were used for further analysis. They were divided into four groups: nulliparous women in spontaneous (n = 9) or oxytocin-induced labor (n = 26), and parous women in spontaneous (n = 11) or oxytocininduced labor (n = 16). Analgesic medication (75 mg pethidine) was administered on request by the patient. 2.2. Instrumentation The procedure of ultrasonic cervimetry has been described in detail elsewhere [3]. Accuracy in vivo and repoducibility in vitro were shown to be good. In brief, the membranes were ruptured and an electrode was attached to the fetal scalp for fetal heart rate monitoring. A water-filled, open-tip pressure catheter was introduced into the uterine cavity through the cervix. The electrode and the intrauterine catheter were connected to a Hewlett Packard 80300 fetal monitor. Two coiled cervimeter transducers measuring 8 x 3.5 x 3.5 mm were screwed into the lateral rims of the cervix at 3 and 9 o’clock, respectively, and connected to the cervimeter. Cervical dilatation, intrauterine pressure and fetal heart rate were recorded simultaneously on a polygraph with a paper speed of 3 cm/h. The electrical outputs of the cervimeter and the tocograph were sampled with a frequency of 1 Hz and stored on magnetic tape for off-line analysis. 2.3. Signal analysis In order to eliminate the high frequency noise component in the low-frequency signal, low-pass filtering was performed before analysis. Both intrauterine pressure and cervical dilatation signals are characterized by a constant or slowly rising resting level interrupted by episodic wave-like elevations. Because of the similarity in shape of pressure and dilatation signals, an identical principle of analysis, based on the first and second derivative, could be applied. A predetermined threshold of the first derivative of the filtered signal of the in-
123-127
trauterine pressure curve of 0.2 mmHg/s and of the cervical dilatation curve of 0.007 cm/s was considered to mark the beginning and end of the signal changes. The gain (cm) in cervical dilatation during a myometrial contraction will be referred to as the cervical dilatation reaction. The cervical reaction time is defined as the time (s) between the onset of a myometrial contraction and the start of the corresponding cervical reaction. The reaction point is defined as the cervical dilatation (cm) at which a cervical reaction occurs for the first time. The acceleration point marks the end of the latent phase and the beginning of the cervical acceleration phase and is defined by the dilatation (cm) at which the second derivative of the arctangens of the filtered dilatation signal reaches the value 0. Mean contraction work per cm of cervical dilatation is estimated by summation of all active pressure areas (mmHg. s) over 1 cm of cervical dilatation. 2.4. Statistical analysis Data are presented as medians (range) or arithmetic means (f SD.), as appropriate. Differences in relative frequencies between groups were analysed with the Fisher test. We used Student’s t-test and Wilcoxon’s rank-sign test (two tailed) to assess differences, and linear regression analysis to evaluate relationships between measured continuous variables. A P value of 0.05 was taken as the level of statistical significance. 3. Results Characteristics of the 62 parturients studied are summarized in Table 1. There were no significant differences in clinical characteristics between groups. We analysed 4800 myometrial contractions, and 3379 cervical dilatation reactions. The significant differences in recorded contractions and dilatation reactions between groups are caused by the smaller number of patients and the shorter duration of the first stage of labor in women with a spontaneous onset of labor compared with women in whom labor was electively induced. In general, the dilatation curves showed a latent phase followed by an acceleration phase characterized by a fast augmentation of cervical dilation (Fig. 1). Quantative data on cervical dilatation patterns are summarized in Table 2. A dilatation reaction was present in all women with spontaneous labor at the start of the recording, and for that reason the cervical reaction point could not be determined in this group. In nulliparous women in whom labor was induced, the reaction point occurred at a significantly greater cervical dilatation than in parous parturients. In the majority of nulliparous women dilatation reactions remained identical during the first stage of labor, whereas an increase in the magnitude of the dilatation reactions was observed in approximately half the number of parous
H.J.H.M.
van Dessel et al. /Eur. J. Ohslet. Gynecol. Reprod. Biol. 53 (1994)
Table I Characteristics
and numbers
of contractions
and cervical
Nulliparous
women
Spontaneous (n = 9)
labor
Maternal age (years) Gestational age (wks)
28 (17-33) 40 (38-42)
Cervix score Birthweight (g) Contractions (n) Dilatation reactions
(n)
8 (3-8) 3335 (2500-3750) 469 377
“Results
(range)
are median
dilatation
123-127
reactions
125
in the four study group? Parous
women
Induced labor (n = 26)
Spontaneous (n= 11)
23 (17-33) 39.5 (38-42) 7 (5-8) 3185 (2590-4130) 2565 1913
28 (23-37) 40 (38-42) 8 (7-8) 3435 (2530-3935) 280 283
labor
Induced labor (n = 16) 28 (I 9-40) 40 (38-42) 6.5 (4-8) 3455 (2430-4215) 1486 806
or n values.
ation point in all groups studied (Table 3). The mean contraction work in parous women with induced labor was approximately 60% of that in nulliparous women with induced labor, before as well as after the acceleration point. The speed of cervical dilatation before the acceleration point was similar in all groups, whereas after the acceleration point it appeared to be significantly increased in parous women with induced labor.
women. None of the parturient women studied showed a significant alteration in cervical reaction time in the course of the first stage of labor. The eficiency of contractions, expressed as the quotient of the dilatation reaction and the active pressure area, did not change as labor progressed. In nulliparas the acceleration point appeared to be similar in spontaneous and in oxytocininduced labor. All parous women in spontaneous labor were apparently already past the acceleration point at the beginning of the recording. The area of active uterine pressure remained constant during labor in all groups. The mean contraction work/cm of cervical dilatation before the acceleration point was approximately 2.5 times that after the acceler-
4. Discussion In contrast to published experience with various types of (electro)mechanical cervimeters, ultrasound cervimetry allows continuous, accurate and precise mea-
10 i t--
latent
phase
->
tacceleration
phase
I
,’ I / , / / / /
/
/’
/’ _---’
/
I’ I’
i
time Fig. I. The dilatation
(hr)
curve. The panels on the right show the original
tracings
at the time indicated
by the dotted
line.
H.J.H.M.
126 Table 2 Variables
of cervical
dilatation
van Dessel et al. /Eur. J. Ohsret. Gvnecol. Reprod. Biol. 53 (1994)
123-127
in the four study groups” Nulliparous
women
Spontaneous (n = 9)
labor
Parous
women
Induced labor (n = 26)
Spontaneous (n = 11)
labor
Induced labor (n = 16)
Dilatation at start (cm) of recording
3.4 * 0.5
3.3 f 0.1
4.0 f 0.3
2.1 f 0.2
Duration of recording (h) Reaction point (cm) Acceleration point (cm)
2.8 f 0.7
5.1 l 0.4 3.6 zt 0.2b 4.8 f 0.2e
1.2 f 0.2
4.2 f 0.6 2.9 f 0.1’ 3.4 f 0.2f
4.5
l
0.4d
“Results are mean values f S.E.M. b vs. cP < 0.05. d.e vs. ‘P < 0.05.
surement of cervical dilatation and the application of the instrument carries little discomfort to the patient [S]. Continuous automated recording of dilatation data allows detection of the reaction and acceleration points, thus defining the first stage of labor. Following a thorough investigation of its somatic, obstetric and psychological consequences, elective induction of labor has become an accepted procedure in our Department [7,9]. This enabled us to investigate cervical dilatation patterns in the early first stage of labor in healthy women without obstetric complications. Other investigators have defined the beginning of the acceleration phase as the cervical dilatation at which the speed of dilatation increases to a fixed value, e.g. to more than
Table 3 Contraction
work per cm of cervical
dilatation
Contraction work per cm before the acceleration point (mmHg* s 103) Contraction work per cm after the acceleration point (mmHg* s IO’) Ratio of contraction work before vs. after acceleration point Dilatation speed before the acceleration point (cmh) Dilatation
speed after the acceleration
(cm/h) Ratio of dilatation eration point
point
speed before vs. after accel-
aResults are means f S.E.M. (range). b vs. cP < 0.05. d vs. eP < 0.05. f,s vs. hNo significant difference. id vs. kP < 0.05.
and dilatation
1.2 cm/h in nulliparous and more than 1.5 cm/h in parous women [lo]. Considering the shape of the dilatation curve, the mathematical method of using the first and second derivative seems to be more correct. The difference between the methods used precludes a comparison between our results and those obtained by others. The dilatation curves obtained in this study show patterns comparable with the sigmoid-shaped curves described in the original work of Friedman [ 111, but we, like others [12], have never observed a deceleration phase. The deceleration phase may be an artefact due to the subtraction method used in the digital assessment of advanced cervical dilatation. Initially, the examiner estimates the distance between the palpating lingers,
speed before and after the acceleration
Nulliparous
women
Spontaneous (n = 9)
labor
pointpa
Parous
women
Induced labor (n = 26)
Spontaneous (n= II)
labor
Induced labor (n = 16)
55.2 l 1.1 (28.0-108.5) 23.4 zt 3.7 (9.7-46.2) 2.36
81.8 f 7.3b (16.8-150.9) 30.5 f 2.5d (I I .8-87.6) 2.68
-
50.6 f 6.0c (19.2-95.5) 20.7 f l.6e (7.5-55.7) 2.44
0.7 f 0.1’ (0.2- 1.3) 2.9 + 0.4’ (1.0-5.1) 0.24
0.5 * 0.1s (0.1-1.0) 2.4 zt 0.2j (0.8-5.3) 0.21
-
0.6 f O.lh (0. I-0.6) 3.5 l 0.2’ (0.9-7.1) 0.17
H.J. H.M. van Dessel et al. /Eur. J. Obstet. Gynecol. Reprod. Biol. 53 (1994)
whereas later, when dilatation exceeds 6 cm or more, the examiner cannot feel the opposing rims anymore and subtracts the remaining rim from 10 cm. The results of our study indicate the occurrence of important functional changes in the uterine cervix during the first stage of labor. First, in oxytocin-induced labor cervical reactions to myometrial contractions appear at 3-4 cm of cervical dilatation. These dilatation reactions can be considered an indicator of increasing cervical elasticity. Second, the finding that more contraction work is needed for 1 cm of dilatation before than after the acceleration point suggests the occurrence of changes in the cervix with the acceleration point as a landmark. Third, the observation that acceleration of cervical dilatation occurs at an earlier stage and myometrial work is less in parous than in nulliparous women with induced labor indicates different cervical behavior due to the process of labor and delivery. However, parous women in induced and nulliparous women in spontaneous labor, do not differ in this respect. In contrast to the findings of Siener [13], we could not demonstrate a significant decrease in cervical reaction time during the course of the first stage of labor. No histological or biochemical data are available to explain the functional differences between the cervices of parous and nulliparous women. The recently proposed hypothesis that the course of the latent phase is largely determined by prostaglandin-mediated proteoglycan changes in contrast to the collagenolysis-dependant acceleration phase [14], may offer a new perspective to understand the differences in cervical behavior between parous and nulliparous parturients. Our results show that ultrasound cervimetry can be used for the study of the physiology and pathophysiology of cervical behavior during labor. Its clinical application awaits the development of appropriate software for on-line analysis of variables of cervical dilatation.
123-127
127
References 1
2
3
4
5
6
7
8
9
10 11 12 13 14
Tuffnell DJ, Bryce F, Johnson N, Lilford RJ. Simulation of cervical changes in labour: reproducibility of expert assessment. Lancet 1989; ii: 1089-1090. Eijskoot F, Storm J, Kok ITh, Wallenburg HC, Wladimiroff JW. An ultrasonic device for continuous measurement of cervical dilatation during labor. Ultrasonics 1977; 55: 183- 185. Kok FTh, Wallenburg HC, Wladimiroff JW. Ultrasonic measurement of cervical dilatation during labor. Am J Obstet Gynecol 1976; 126: 288-290. Zador I, Neuman MR, Wolfson RN. Continuous monitoring of cervical dilatation during labour by ultrasonic transmit-time measurement, Med Biol Eng 1976; 14: 299-305. Moss PL, Lauron P, Roux JF, Neuman MR, Dmytrus KC. Continuous cervical dilatation monitoring by ultrasonic methods during labor. Am J Obstet Gynecol 1978; 132: 16-19. Burnhill MS, Danezis J, Cohen J. Uterine contractility during labor studied by intraamniotic fluid pressure recordings. Part 1. Effect of age, parity, duration of pregnancy, quality of the cervix, sedation, position, dose level, and amount of oxytocics on the course of labor. Am J Obstet Gynecol 1962; 83: 561-571. Vierhout ME, Oud JJ, Wallenburg HCS. Elective induction of labor: a prospective clinical study. I: Obstetric and neonatal effects. J Perinat Med 1985; 13: 155-162. van Dessel T, Frijns JH, Kok FT, Wallenburg HC. Assessment of cervical dilatation during labor: a review. Eur J Obstet Gynecol Reprod Biol 1991; 41: 165-171. Out JJ, Vierhout ME, Verhage F, Duivenvoorden HJ, Wallenburg HCS. Elective induction of labor: a prospective clinical study. Part II: Psychological effects. J Perinat Med 1985; 13: 163-170. Peisner DB, Rosen MC. Transition from latent to active labor. Obstet Gynaecol 1986; 68: 448-451. Friedman EA. Graphic analysis of labor. Am J Obstet Gynecol 1954; 68: 1568-1575. Studd J. The partographic control of labour. Clin Obstet Gynecol 1975; 2: 127-151. Siener H. Cervical dynamometry, a new method in obstetrical research. Am J Obstet Gynecol 1964; 89: 579-582. Rath W, Osmers R, Adelmann-Grill BC, Stuhlsatz HW, Tschesche H. Szevdirini M. Grundlagen der physiologischen und medikamentds induzierten Zervixreifung Neuere morphologische und biochemische Befunde. Geburtshilfe Frauenheilkd 1990: 50: 657-664.
Ultrasound assessment of cervical dynamics during the first stage of labor H.J.H.M. van Dessel*a, J.H.M. Frijnsb, F.Th.J.G.Th.
Kok”, H.C.S. Wallenburg”
‘Institute of Obstetrics and Gynecology, Erasmus University School of Medicine and Health Sciences, EE 2283, P. 0. Box 1738. 300 DR Rotterdam, The Netherlands bDepartment of Otolaryngology, University Hospital, Leiden. The Netherlancis ‘Department of Obstetrics and Gynecology, Westeinde Hospital, The Hague, The Netherlands
(Accepted 3 November 1993)
Abstract Objective: Assessment of cervical dynamics during the first stage of labor by a combination of ultrasound cervimetry and intrauterine tocography. Methods: Sixty-two parturients were divided into four groups: nulliparous women in spontaneous (n = 9) or oxytocin-induced labor (n = 26), parous women in spontaneous (n = 11) or oxytocin-induced labor (n = 16). Intrauterine pressure and cervical dilatation were continuously recorded and assessed by off-line computer analysis. Results: All women with spontaneous labor showed cervical responses to uterine contractions at the beginning of the recording. The first cervical response to a uterine contraction occurred at a significantly smaller dilatation in parous than in nulliparous women with induced labor (2.9 and 3.6 cm, respectively). Also, acceleration of cervical dilatation occurred at less dilatation in parous than in nulliparous women (3.4 cm and 4.8 cm, respectively), and myometrial work per cm of cervical dilatation was less in parous than in nulliparous parturients. Conclusions: The results indicate significant differences between cervical dilatation patterns in nulliparous and parous women, which may be due to structural cervical changes caused by labor and parturition. The labor patterns found were different from those originally described by Friedman (Friedman EA. Graphic analysis of labor. Am J Obstet Gynecol 1954; 68: 1568-1575), as no deceleration phases were detected. Ultrasound cervimetry is a valuable technique for the study of cervical dynamics during labor. Key words:
Ultrasound
cervimetry; Cervical dilatation;
First stage of labor
1. Introduction Although digital assessment of cervical dilatation is generally applied in clinical obstetrics, the intermittent nature of the procedure and its low accuracy and precision limit its usefulness for the study of cervical behavior during labor [ 11. In earlier reports we described a device for continuous measurement of cervical dilatation using ultrasound [2,3]. A similar device has been reported by others [4,5]. In combination with a tocograph, the ultrasound cervimeter provides continuous quantative data on cervical dynamics during labor. It consists of two small ultrasonic transducers attached to the lateral rims of the dilating cervix and connected to an electronic processing unit. Ultrasound bursts are transmitted by one transducer and received by the other one, and the dis-
tance between the transducers is calculated electronically from the transmission time. Using this technique we studied the dynamics of the human cervix during the first stage of spontaneous and oxytocin-induced labor with the aim of assessing cervical dilatation patterns in association with intrauterine pressure. 2. Material and methods 2.1. Patients Subjects were recruited from pregnant women under obstetric care in the antenatal clinic of the AZR-Dijkzigt Hospital. The women selected met all of the following criteria: (i)
* Corresponding
author.
0028-2243/94/$07.00 0 1994 Elsevier Science Ireland SSDI 0028-2243(93)01733-A
Ltd. All rights reserved.
uncomplicated pregnancy cephalic position;
with a single fetus in
124
H. J. H. M. van Dessel et al. /Eur. J. Obstet. Gynecol. Reprod. Biol. 53 (1994)
(ii) duration of pregnancy between 38 and 42 weeks; and (iii) admission to the labor room in the first stage of spontaneous labor or for scheduled elective induction of labor. Spontaneous labor was defined as the occurrence of regular painful contractions every 3-5 min in a woman with a cervix score according to Burnhill et al. of 6 or more [6]. Elective induction of labor was defined as induction of labor in the absence of a medical-obstetric indication for termination of pregnancy [7]. Elective induction is an accepted procedure in our Department, and was performed in the presence of a cervix score of 6 or more only. Sixty-seven term parturients gave their informed oral consent and were enrolled in the study. Of the 67 recordings, live were of insufficient quality, mainly due to early disconnection of the cervical transducers; 62 recordings were used for further analysis. They were divided into four groups: nulliparous women in spontaneous (n = 9) or oxytocin-induced labor (n = 26), and parous women in spontaneous (n = 11) or oxytocininduced labor (n = 16). Analgesic medication (75 mg pethidine) was administered on request by the patient. 2.2. Instrumentation The procedure of ultrasonic cervimetry has been described in detail elsewhere [3]. Accuracy in vivo and repoducibility in vitro were shown to be good. In brief, the membranes were ruptured and an electrode was attached to the fetal scalp for fetal heart rate monitoring. A water-filled, open-tip pressure catheter was introduced into the uterine cavity through the cervix. The electrode and the intrauterine catheter were connected to a Hewlett Packard 80300 fetal monitor. Two coiled cervimeter transducers measuring 8 x 3.5 x 3.5 mm were screwed into the lateral rims of the cervix at 3 and 9 o’clock, respectively, and connected to the cervimeter. Cervical dilatation, intrauterine pressure and fetal heart rate were recorded simultaneously on a polygraph with a paper speed of 3 cm/h. The electrical outputs of the cervimeter and the tocograph were sampled with a frequency of 1 Hz and stored on magnetic tape for off-line analysis. 2.3. Signal analysis In order to eliminate the high frequency noise component in the low-frequency signal, low-pass filtering was performed before analysis. Both intrauterine pressure and cervical dilatation signals are characterized by a constant or slowly rising resting level interrupted by episodic wave-like elevations. Because of the similarity in shape of pressure and dilatation signals, an identical principle of analysis, based on the first and second derivative, could be applied. A predetermined threshold of the first derivative of the filtered signal of the in-
123-127
trauterine pressure curve of 0.2 mmHg/s and of the cervical dilatation curve of 0.007 cm/s was considered to mark the beginning and end of the signal changes. The gain (cm) in cervical dilatation during a myometrial contraction will be referred to as the cervical dilatation reaction. The cervical reaction time is defined as the time (s) between the onset of a myometrial contraction and the start of the corresponding cervical reaction. The reaction point is defined as the cervical dilatation (cm) at which a cervical reaction occurs for the first time. The acceleration point marks the end of the latent phase and the beginning of the cervical acceleration phase and is defined by the dilatation (cm) at which the second derivative of the arctangens of the filtered dilatation signal reaches the value 0. Mean contraction work per cm of cervical dilatation is estimated by summation of all active pressure areas (mmHg. s) over 1 cm of cervical dilatation. 2.4. Statistical analysis Data are presented as medians (range) or arithmetic means (f SD.), as appropriate. Differences in relative frequencies between groups were analysed with the Fisher test. We used Student’s t-test and Wilcoxon’s rank-sign test (two tailed) to assess differences, and linear regression analysis to evaluate relationships between measured continuous variables. A P value of 0.05 was taken as the level of statistical significance. 3. Results Characteristics of the 62 parturients studied are summarized in Table 1. There were no significant differences in clinical characteristics between groups. We analysed 4800 myometrial contractions, and 3379 cervical dilatation reactions. The significant differences in recorded contractions and dilatation reactions between groups are caused by the smaller number of patients and the shorter duration of the first stage of labor in women with a spontaneous onset of labor compared with women in whom labor was electively induced. In general, the dilatation curves showed a latent phase followed by an acceleration phase characterized by a fast augmentation of cervical dilation (Fig. 1). Quantative data on cervical dilatation patterns are summarized in Table 2. A dilatation reaction was present in all women with spontaneous labor at the start of the recording, and for that reason the cervical reaction point could not be determined in this group. In nulliparous women in whom labor was induced, the reaction point occurred at a significantly greater cervical dilatation than in parous parturients. In the majority of nulliparous women dilatation reactions remained identical during the first stage of labor, whereas an increase in the magnitude of the dilatation reactions was observed in approximately half the number of parous
H.J.H.M.
van Dessel et al. /Eur. J. Ohslet. Gynecol. Reprod. Biol. 53 (1994)
Table I Characteristics
and numbers
of contractions
and cervical
Nulliparous
women
Spontaneous (n = 9)
labor
Maternal age (years) Gestational age (wks)
28 (17-33) 40 (38-42)
Cervix score Birthweight (g) Contractions (n) Dilatation reactions
(n)
8 (3-8) 3335 (2500-3750) 469 377
“Results
(range)
are median
dilatation
123-127
reactions
125
in the four study group? Parous
women
Induced labor (n = 26)
Spontaneous (n= 11)
23 (17-33) 39.5 (38-42) 7 (5-8) 3185 (2590-4130) 2565 1913
28 (23-37) 40 (38-42) 8 (7-8) 3435 (2530-3935) 280 283
labor
Induced labor (n = 16) 28 (I 9-40) 40 (38-42) 6.5 (4-8) 3455 (2430-4215) 1486 806
or n values.
ation point in all groups studied (Table 3). The mean contraction work in parous women with induced labor was approximately 60% of that in nulliparous women with induced labor, before as well as after the acceleration point. The speed of cervical dilatation before the acceleration point was similar in all groups, whereas after the acceleration point it appeared to be significantly increased in parous women with induced labor.
women. None of the parturient women studied showed a significant alteration in cervical reaction time in the course of the first stage of labor. The eficiency of contractions, expressed as the quotient of the dilatation reaction and the active pressure area, did not change as labor progressed. In nulliparas the acceleration point appeared to be similar in spontaneous and in oxytocininduced labor. All parous women in spontaneous labor were apparently already past the acceleration point at the beginning of the recording. The area of active uterine pressure remained constant during labor in all groups. The mean contraction work/cm of cervical dilatation before the acceleration point was approximately 2.5 times that after the acceler-
4. Discussion In contrast to published experience with various types of (electro)mechanical cervimeters, ultrasound cervimetry allows continuous, accurate and precise mea-
10 i t--
latent
phase
->
tacceleration
phase
I
,’ I / , / / / /
/
/’
/’ _---’
/
I’ I’
i
time Fig. I. The dilatation
(hr)
curve. The panels on the right show the original
tracings
at the time indicated
by the dotted
line.
H.J.H.M.
126 Table 2 Variables
of cervical
dilatation
van Dessel et al. /Eur. J. Ohsret. Gvnecol. Reprod. Biol. 53 (1994)
123-127
in the four study groups” Nulliparous
women
Spontaneous (n = 9)
labor
Parous
women
Induced labor (n = 26)
Spontaneous (n = 11)
labor
Induced labor (n = 16)
Dilatation at start (cm) of recording
3.4 * 0.5
3.3 f 0.1
4.0 f 0.3
2.1 f 0.2
Duration of recording (h) Reaction point (cm) Acceleration point (cm)
2.8 f 0.7
5.1 l 0.4 3.6 zt 0.2b 4.8 f 0.2e
1.2 f 0.2
4.2 f 0.6 2.9 f 0.1’ 3.4 f 0.2f
4.5
l
0.4d
“Results are mean values f S.E.M. b vs. cP < 0.05. d.e vs. ‘P < 0.05.
surement of cervical dilatation and the application of the instrument carries little discomfort to the patient [S]. Continuous automated recording of dilatation data allows detection of the reaction and acceleration points, thus defining the first stage of labor. Following a thorough investigation of its somatic, obstetric and psychological consequences, elective induction of labor has become an accepted procedure in our Department [7,9]. This enabled us to investigate cervical dilatation patterns in the early first stage of labor in healthy women without obstetric complications. Other investigators have defined the beginning of the acceleration phase as the cervical dilatation at which the speed of dilatation increases to a fixed value, e.g. to more than
Table 3 Contraction
work per cm of cervical
dilatation
Contraction work per cm before the acceleration point (mmHg* s 103) Contraction work per cm after the acceleration point (mmHg* s IO’) Ratio of contraction work before vs. after acceleration point Dilatation speed before the acceleration point (cmh) Dilatation
speed after the acceleration
(cm/h) Ratio of dilatation eration point
point
speed before vs. after accel-
aResults are means f S.E.M. (range). b vs. cP < 0.05. d vs. eP < 0.05. f,s vs. hNo significant difference. id vs. kP < 0.05.
and dilatation
1.2 cm/h in nulliparous and more than 1.5 cm/h in parous women [lo]. Considering the shape of the dilatation curve, the mathematical method of using the first and second derivative seems to be more correct. The difference between the methods used precludes a comparison between our results and those obtained by others. The dilatation curves obtained in this study show patterns comparable with the sigmoid-shaped curves described in the original work of Friedman [ 111, but we, like others [12], have never observed a deceleration phase. The deceleration phase may be an artefact due to the subtraction method used in the digital assessment of advanced cervical dilatation. Initially, the examiner estimates the distance between the palpating lingers,
speed before and after the acceleration
Nulliparous
women
Spontaneous (n = 9)
labor
pointpa
Parous
women
Induced labor (n = 26)
Spontaneous (n= II)
labor
Induced labor (n = 16)
55.2 l 1.1 (28.0-108.5) 23.4 zt 3.7 (9.7-46.2) 2.36
81.8 f 7.3b (16.8-150.9) 30.5 f 2.5d (I I .8-87.6) 2.68
-
50.6 f 6.0c (19.2-95.5) 20.7 f l.6e (7.5-55.7) 2.44
0.7 f 0.1’ (0.2- 1.3) 2.9 + 0.4’ (1.0-5.1) 0.24
0.5 * 0.1s (0.1-1.0) 2.4 zt 0.2j (0.8-5.3) 0.21
-
0.6 f O.lh (0. I-0.6) 3.5 l 0.2’ (0.9-7.1) 0.17
H.J. H.M. van Dessel et al. /Eur. J. Obstet. Gynecol. Reprod. Biol. 53 (1994)
whereas later, when dilatation exceeds 6 cm or more, the examiner cannot feel the opposing rims anymore and subtracts the remaining rim from 10 cm. The results of our study indicate the occurrence of important functional changes in the uterine cervix during the first stage of labor. First, in oxytocin-induced labor cervical reactions to myometrial contractions appear at 3-4 cm of cervical dilatation. These dilatation reactions can be considered an indicator of increasing cervical elasticity. Second, the finding that more contraction work is needed for 1 cm of dilatation before than after the acceleration point suggests the occurrence of changes in the cervix with the acceleration point as a landmark. Third, the observation that acceleration of cervical dilatation occurs at an earlier stage and myometrial work is less in parous than in nulliparous women with induced labor indicates different cervical behavior due to the process of labor and delivery. However, parous women in induced and nulliparous women in spontaneous labor, do not differ in this respect. In contrast to the findings of Siener [13], we could not demonstrate a significant decrease in cervical reaction time during the course of the first stage of labor. No histological or biochemical data are available to explain the functional differences between the cervices of parous and nulliparous women. The recently proposed hypothesis that the course of the latent phase is largely determined by prostaglandin-mediated proteoglycan changes in contrast to the collagenolysis-dependant acceleration phase [14], may offer a new perspective to understand the differences in cervical behavior between parous and nulliparous parturients. Our results show that ultrasound cervimetry can be used for the study of the physiology and pathophysiology of cervical behavior during labor. Its clinical application awaits the development of appropriate software for on-line analysis of variables of cervical dilatation.
123-127
127
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6
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9
10 11 12 13 14
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