Onset and outcomes of spontaneous labour in low risk nulliparous women.

Objectives: The objective of this study was to: 1. Establish the median gestational age of spontaneous labour for low-risk nulliparas. 2. Examine the variation in mode of delivery and short-term neonatal outcomes with gestation at onset of spontaneous labour.

Study Design: This is a retrospective observational cohort study conducted at a tertiary obstetric unit. The study population was 12, 323 low risk nulliparous women with singleton pregnancies who experienced spontaneous onset of labour. The study period was over seven years, from Jan 1st 2011 to 31st Dec 2017. Exclusion criteria were multiparity, multi-fetal pregnancy, booking after 14 weeks gestation, antepartum or intrapartum death, or any obstetric or fetal indication for delivery with the exception of post-maturity. Gestation of onset of spontaneous labour, demographic variables and maternal and neonatal outcomes were collected. The primary outcome was median gestational age at onset of spontaneous labour and its distribution at term. Secondary outcomes were mode of delivery and neonatal outcomes including low-apgar score and NICU admission.

Results: 12, 323 patients were eligible for inclusion. Median gestation for onset of labour was 40.1 weeks gestation, with 80.5% of spontaneous labour occurs by 41 + 0 weeks gestation. The risk of assisted delivery (RR 1.32, 95% CI 1.23 - 1.42), caesarean section (RR 2.17, 95% CI 1.88-2.51) and low-apgar scores (RR 3.13 95% CI 1.50-6.55) increased significantly with spontaneous labour after 41 weeks' gestation.

Conclusions: Nulliparous women with low-risk pregnancies are most likely to experience spontaneous labour between 40 + 0 and 40 + 6. 80.5% of spontaneous labour occurred by 41 + 0 weeks gestation. Assisted vaginal delivery, caesarean section and low-apgar scores were significantly more likely with spontaneous labour after 41 weeks' gestation.