Recently receiving much interest in the field of obstetric ultrasound are the assessment of its accuracy to predict spontaneous onset of labor at term, the success rate of induction of labor and the risk of Cesarean section after onset of spontaneous or induced labor. Two main factors drive current research. (i) Although the Bishop score1 remains the standard method for predicting the duration and safety of induced labor, digital cervical examination is subjective2 and the Bishop score may be of limited value in women with a low score3, 4. Ultrasound assessment of the cervix is more objective and reproducible5. (ii) The ultrasonographic measurement of cervical length is predictive of delivery before 37 weeks of gestation, the risk of spontaneous preterm delivery increasing as the ultrasonographic length of the cervix decreases6-8. Numerous studies, therefore, have compared ultrasonographic measurement of cervical length with digital cervical examination in the prediction of the time interval to delivery and the incidence of vaginal delivery in patients undergoing induction of labor9-20. Their results, however, are contradictory, probably because of the numerous weaknesses and/or differences in their methodologies (e.g. heterogeneous populations, including variable proportions of patients with an unripe cervix, absence of standardization of the method of induction (amniotomy and/or oxytocin and/or different prostaglandins), lack of power, secondary exclusion of patients) and the multiple and different endpoints used to evaluate success or failure of induction (e.g. total Cesarean section rate or vaginal delivery rate, duration of latent phase or total duration of labor, vaginal delivery within 24 h of induction, rate of active labor within 12 h, Cesarean section rate before dilatation reaches 8 cm). The different parameters used to assess the cervix both clinically (Bishop score or one of its components) and ultrasonographically (cervical length, dilatation, presence of wedging or funneling, cervical angle) also contribute to the differing results. Recently, Hatfield et al.21 reviewed the literature and performed a meta-analysis, evaluating sonographic cervical assessment to predict successful induction of labor; they concluded that it is not an effective predictor of successful labor induction and that it cannot predict any specific outcome (such as mode of delivery). However, studies investigating the usefulness of sonographic assessment of cervical length in the prediction of spontaneous onset of labor in low-risk women enrolled at 37 or more weeks of gestation consistently report more encouraging results. Seven studies22-28, including a total of 2308 patients, found cervical length measurement to be a significant predictor of the likelihood of the onset of spontaneous labor and/or of delivery within 7 days and/or delivery by 41 weeks. Overall, therefore, the published data show that ultrasonographic measurement of cervical length is a useful tool to predict imminent spontaneous delivery in preterm and term births, but that it is ineffective in predicting the success or failure of induction of labor. These apparently discordant results are in fact convergent: while cervical length is not a marker of cervical ripening, it may well be a marker of myometrial ripeness. The processes of cervical ripening and of uterine activity are independent29. Cervical and myometrial ripeness are often correlated, but this is not necessarily always the case; thus, it is possible that myometrial ripeness may sometimes precede cervical ripening and, conversely, that uterine activity is not coordinated with physiological preparedness for labor. We have shown previously22 that ultrasound measurement of cervical length between 39 + 4 weeks and 40 + 3 weeks of gestation is predictive of the onset of spontaneous labor within 7 days, while fetal fibronectin (fFN) assay is useful in predicting the risk of Cesarean section. We hypothesized that ultrasound examination of the cervix and fFN assay addressed two different physiological processes involved in parturition. In predicting the time to the onset of spontaneous labor, cervical length may be a marker of myometrial ripeness, whereas fFN is a confirmed marker of cervical ripening. Furthermore, the absence of preliminary myometrial ripeness among patients requiring prostaglandins for induction of labor may explain the absence of correlation between cervical length and induced labor duration18. Clearly, other sonographic approaches are necessary to predict the success rate of induction of labor and the risk of Cesarean section after induction. Eggebo et al.30 in this issue of the Journal contribute to this area of research, by evaluating fetal head–perineum distance as a predictive factor for vaginal delivery after induction of labor. Although its predictive value was similar to that of ultrasonographically measured cervical length and Bishop score, the main strength of this study is to open our minds towards the investigation of new approaches to better predict the outcome of induction of labor when the cervix is unripe. As unsatisfying as it may be, we must concede that the cervix has secrets as yet undiscovered by ultrasound.
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Open Access
Apr 25, 2023
Monica Cruz-Lemini Md, Phd + 6
