Objective
The objective of the study was to evaluate the clinical, sonographic, and hormonal variables that influence the success of labor induction in nulliparous postterm pregnancies.
Study Design
Fifty nulliparous women with a single postterm pregnancy receiving a slow-release prostaglandin estradiol pessary were prospectively enrolled, and clinical characteristics were analyzed in relation to success of induction of labor. Clinical, sonographic, and hormonal variables were analyzed by univariate statistical analysis and multivariate logistic regression for the prediction of successful induction.
Results
The group of patients delivering within 24 hours differed significantly from the remaining patients by higher Bishop scores, body mass indices, estradiol serum concentrations, estriol to estradiol ratios, and shorter cervices. The combination of cervical length and estriol to estradiol ratio achieved a sensitivity of 100% (95% confidence interval, 71.3–100%) and a specificity of 94.1% (95% confidence interval, 80.3–99.1%).
Conclusion
Cervical length and the estriol to estradiol ratio represent good predictive indicators of the response to the induction of labor in postterm pregnancies.
Postterm pregnancy is defined as a pregnancy that continues to or beyond 42 weeks (294 days) from the first day of the last normal menstrual period or 14 days beyond the best obstetric estimate of the date of delivery. It represents approximately 7% of all pregnancies, and the prevalence depends on the population characteristics, including such factors as nulliparity, a prior postterm pregnancy, and genetic predisposition. Among etiologic factors, the most common cause of postterm pregnancy is an error in gestational age dating, but in the majority of other cases, the cause is unknown.
The main complications associated with pregnancy beyond 40 and 41 weeks of gestation are stillbirth, perinatal mortality and morbidity, and increased risk of cesarean delivery. Indeed, the most appropriate prevention and management of postterm pregnancy include accurate gestational age assessment in early pregnancy, antenatal fetal surveillance, and the timely initiation of delivery if spontaneous labor does not occur. In fact, obstetricians favor a policy of routine induction of labor for low-risk pregnancies at 41 weeks of gestation, and it appears to be an effective strategy to reduce the risk of late intrauterine death. Among pharmacologic options, 1 of the most common drugs used as ripening agents in patients with an unfavorable cervix is the dinoprostone (prostaglandin E2 [PGE2]) vaginal insert, approved in 1995 by the Food and Drug Administration, that has been shown to be effective in shortening the interval between induction and vaginal delivery safely.
Successful induction has been defined as vaginal delivery within 24 hours after induction. Current practice in nulliparous women following inductions results in an instrumental delivery in almost 50% of patients and an overall cesarean section rate as high as 22%. Because of the risk of failed induction of labor, a variety of maternal and fetal factors as well as screening tests have been used to predict the response to induction, and among them parity and ultrasound measurements of cervical length have been shown to be the most predictive.
Estrogens have been demonstrated to be important hormones involved in the regulation of several functions during pregnancy. In particular, estriol (E3), estradiol (E2), and their ratio play an important role in the control of parturition, creating a specific estrogenic environment at the onset of labor. On the basis of these considerations, we hypothesized that estrogens may contribute to a better assessment of women at risk for an unsuccessful induction of labor in postterm pregnancy.
In the present study, we evaluated statistically maternal clinical characteristics, ultrasound cervical length measurement, and serum estrogens E3 and E2 and their ratios as potential variables influencing the response to the induction of labor with a vaginal PGE2 insert in a sample of 50 nulliparous women with postterm pregnancies.
Materials and Methods
This is a prospective study including 50 women with a single postterm pregnancy admitted consecutively to the Department of Pediatrics, Obstetrics, and Reproductive Medicine, Siena University Hospital, from July 2009 to March 2010.
All patients presented an unfavorable cervix (Bishop score <6) and were elective for induction of labor, which was performed by the use of slow-release dinoprostone insert (Propess; Ferring, Copenhagen, Denmark) in accordance with the international guidelines. This insert is a preparation of PGE2 packaged in a hydrogel polymer matrix and designed for slow intravaginal release of 10 mg dinoprostone at a rate of 0.3 mg/h over 12 hours. It is applied in the posterior fornix, with the membranes still intact.
The fetal heart rate was monitored 30 minutes before and 1 hour after dinoprostone administration in all patients. The vaginal insert was removed 24 hours after the application or earlier in case of onset of active labor, rupture of membranes, and abnormal cardiotocography (the occurrence of hyperstimulation or altered fetal heart rate). Oxytocin augmentation was started in the presence of unsatisfactory progress of labor or amniotomy. Both during induction and oxytocin stimulation, fetal heart rate and uterine contractions were monitored by cardiotocography.
Clinical characteristics such as maternal age, Bishop score, cervical length, body mass index (BMI), mode of delivery, and neonatal characteristics such as birthweight were all recorded.
Women with multiple pregnancies, nonreassuring fetal testing, diabetes mellitus, abruptio placenta, preeclampsia, intrauterine growth restriction, fetal anomalies, vaginal bleeding, breech presentations, ruptured membranes, and clinical signs of infection were excluded from the study.
All women were submitted to blood collection and cervical length ultrasound measurement before labor induction. Blood was obtained by venipuncture, transferred to heparin tubes, and centrifuged after 30 minutes from sampling at 4°C (6000 × g for 10 minutes). All plasma samples were stored at –80°C until assay.
Cervical length ultrasound measurement was performed according to previously validated technical criteria, with the use of MyLab50 Esaote (MyLab Family Group Esaote SpA, Genova, Italy) ultrasound scan equipment with a 4.5-7.0 MHz transvaginal probe. We considered as primary outcome vaginal delivery within 24 hours after induction, and outcomes were divided into responders (n = 34) and nonresponders (n = 11) to the induction of labor to perform the statistical analyses.
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