Predicting the spontaneous onset of labour in post-date pregnancies: a population-based retrospective cohort study.

OBJECTIVE To estimate the probability of spontaneous onset of labour (SOL) among women with uncomplicated pregnancies who have reached 41+0 weeks and to examine the influence of maternal characteristics on this event. METHODS We conducted a population-based retrospective cohort study of women with uncomplicated singleton pregnancies in cephalic presentation between 41+0 and 42+0 weeks' gestation. Detailed clinical information was obtained from the British Columbia Perinatal Data Registry. We determined the time from 41+0 weeks to the exact day and time of SOL, pre-labour Caesarean section, or onset of labour following induction. A Kaplan-Meier curve was created to estimate the probability of SOL. A Cox regression model was used to assess the independent influence of maternal age, parity, BMI, and pregnancy weight gain on the SOL, and to assess the extent to which prediction of SOL could be individualized according to a woman's characteristics. RESULTS Among 15 253 women undelivered at 41+0 weeks, there was a 67.6% (95% CI 66.4% to 68.7%) chance of SOL by 41+6 weeks. Although SOL was statistically more likely in younger women, higher parity, lower BMI, and lower weight gain (P<0.01), the multivariable model's predictive ability was poor (c-statistic 0.56). CONCLUSION Maternal characteristics were not a strong determinant for successful individualized prediction of SOL in women with uncomplicated pregnancies reaching 41+0 weeks of gestation. Our population-based estimates of the daily occurrence of SOL can be used to inform discussions with women on when to offer induction of labour.