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

Objective: Universal elective induction of labor (IOL) in singleton parous pregnancies has been advocated to reduce the rate of cesarean section (CD), without impacting on maternal outcome. However, about 50% of women deliver after 40 weeks; therefore, an accurate estimation of the time of delivery might avoid unnecessary early IOL. The aim of this study was to test the diagnostic accuracy of ultrasound in predicting delivery ≥40 weeks of gestation in singleton parous women. Methods: Prospective cohort study of singleton parous women undergoing a dedicated ultrasound assessment at 36–38 weeks of gestation. The primary outcome was spontaneous vaginal delivery ≥40 weeks of gestation. Cervical length (CL), posterior cervical angle (PCA), sonoelastographic hardness ratio (HR), angle of progression (AoP) and head perineal distance (HPD) were measured. Multivariate logistic regression and area under the curve (AUC) analyses were used to test the diagnostic accuracy of different maternal and ultrasound characteristics in predicting delivery ≥40 weeks. Results: 518 singleton pregnancies were included in the analysis and 235 (45.4%) delivered ≥40 weeks. CL (29 vs 19 mm; p ≤.0001) and HPD (50 vs 47 mm; p =.001) were longer, HR higher (38.9 vs 35.5; p =.04), while PCA (98° vs 104°; p ≤.0001) and AOP narrower (93° vs 98°; p =.029) in pregnancies delivered compared to those not delivered after 40 weeks of gestation. At multivariable logistic regression analysis, CL (aOR 1.206; 95% CI 1.164–1.250), HPD (aOR 1.127; 95% CI 1.066–1.191) and HR (aOR 1.022; 95% CI 1.003–1.041 were the only variables independently associated with delivery ≥40 weeks. CL showed had an AUC of 0.863 in predicting delivery ≥40 weeks of gestation, with an optimal cutoff of 23.5 mm. Integration of HPD and HR did not significantly improve the diagnostic performance of CL alone to predict delivery ≥40 weeks (AUC 0.870; p =.472). Conclusion: Cervical length at 36–38 weeks has a good diagnostic accuracy to predict spontaneous vaginal delivery at ≥40 weeks. Universal assessment of CL in the third trimester of pregnancy may help in identifying those women who may benefit of elective IOL at 39 weeks.