Prolonged pregnancy is associated with an increased risk of maternal, fetal and neonatal complications. Careful monitoring and systematic induction at 42 weeks of gestation are recommended in France.

Our primary objective was to identify determinants for spontaneous labor during expectant management for prolonged pregnancy. Our secondary objective was to assess the efficacy of membrane sweeping in bringing on labor.

This observational, retrospective, monocentric study included all patients with a singleton pregnancy who had reached the term of 41 weeks of gestation to within one day. They underwent an expectant period up to 41 weeks and 6 days with fetal heart monitoring and ultrasound every two days. The cohort was divided into two groups according to the presence or absence of membrane sweeping. In the group that did not undergo membrane sweeping (Natural History cohort), we compared patients who entered spontaneous labor with those who required induction of labour.

A total of 366 patients were included. For the 293 patients without membrane sweeping, spontaneous labor occurred in 51.1 % of cases. A higher Bishop score ( p = 0.02) and a lower BMI ( p = 0.08) were associated with spontaneous labor. Parity had no influence ( p = 0.74). Effectiveness of membrane sweeping to promote spontaneous labor was highlighted after adjustment on parity, BMI and Bishop score (71.6 % vs 51.5 %, p = 0.021, SMD=0.422).

In prolonged pregnancy, the probability of going into spontaneous labor was only 50.9 %. Low BMI and high Bishop score were associated with spontaneous onset of labor in case of expectant management. Membrane sweeping was associated with spontaneous labor in our post-term population after adjustment on parity, BMI and Bishop score.