Prediction of delivery after 40 weeks by antepartum ultrasound in singleton nulliparous women: a prospective cohort study

Background: Induction of labor at 39 weeks of gestation is associated with better maternal and perinatal outcomes than expectant management. However, a policy of induction of labor implies the identification of women who will deliver after 40 weeks, who are at higher risk of adverse outcome. Objective: This study primarily aimed to elucidate the role of antepartum ultrasound in predicting the onset of spontaneous labor in a cohort of low-risk singleton pregnancies, and secondarily to compare its diagnostic performance with that of other ultrasonographic and clinical parameters. Study design: This was a prospective study including singleton nulliparous women undergoing a dedicated ultrasound assessment at 36 to 38 weeks of gestation. The primary outcome was delivery ≥40 weeks of gestation. The ultrasound parameters explored were cervical length, posterior cervical angle, angle of progression, and head-perineum distance. Multivariate logistic regression, Kaplan-Meier, and area under the curve analyses were used to test the strength of association and diagnostic performance of variables considered in predicting delivery ≥40 weeks. Results: A total of 457 women were included, and 49.2% delivered ≥40 weeks. Cervical length was longer (30 vs 19 mm; P≤.0001) and posterior cervical angle wider (105° vs 98°, P≤.0001) in women delivering ≥40 weeks than those delivering <40 weeks. Similarly, head-perineum distance was longer (48 vs 40 mm; P=.001) and angle of progression narrower (93° vs 95°; P=.04) in pregnancies delivering after 40 weeks. Conversely, there was no difference in the modified Bishop score between the 2 study groups (P=.689). In multivariable logistic regression analysis, cervical length (adjusted odds ratio, 1.307) and head-perineum distance (adjusted odds ratio, 1.227) were independently associated with delivery ≥40 weeks. Cervical length showed an area under the curve of 0.896 in predicting a delivery after 40 weeks. Integration of head-perineum distance in the diagnostic algorithm did not increase the performance of the model. A cervical length of 24 mm at 36 to 37 weeks of gestation showed the best combination of sensitivity and specificity in predicting delivery ≥40 weeks, with a shorter latency between ultrasound assessment and birth. Conclusion: Antepartum ultrasound can reliably identify a subset of nulliparous women at higher risk of delivering beyond 40 weeks. A cervical length >24 mm at 36 to 37 weeks of gestation shows the optimal combination of sensitivity and specificity in predicting delivery ≥40 weeks. The findings from this study can help in identifying those women for whom elective induction of labor at 39 weeks of gestation would be beneficial in reducing the risk of adverse pregnancy outcome.