Background
There is emerging evidence that caesarean section at full dilatation is associated with an increased risk of subsequent spontaneous preterm birth.

Aim
To investigate the association between caesarean section at full dilatation and spontaneous preterm birth in subsequent pregnancies.

Materials and methods
This was a retrospective cohort study of women who had two consecutive births at Royal Prince Alfred Hospital, 1989–2015. Our main comparison group was women who had emergency caesarean sections during the first stage of labour. Secondary comparison groups were women who had elective caesarean sections, instrumental deliveries and unassisted vaginal deliveries. The primary outcome was spontaneous preterm birth (<37 weeks gestation) in a subsequent pregnancy. Results There were 2672 women who had an emergency caesarean section, with 2142 (80%) performed during the first stage of labour and 533 (20%) at full dilatation. The rates of spontaneous preterm birth in a subsequent pregnancy were 1.7% and 3.8%, respectively (odds ratio 2.2 (95%CI 1.3–3.8), P = 0.003). The hazard ratio for spontaneous onset of labour at any given gestation from 20 weeks until full term was 1.4 (95%CI 1.2–1.6) and did not change after adjusting for maternal age and body mass index. Conclusion There is a significantly higher rate of subsequent spontaneous preterm birth in women who had a caesarean section at full dilatation compared with women who had a caesarean section during the first stage of labour. Awareness of this as a risk factor may warrant referral to a high‐risk obstetric or preterm birth clinic.