Previous preterm birth is a strong predictor of subsequent preterm birth, but less is known about the causes of preterm birth following a full‐term first pregnancy. Recent research has highlighted previous caesarean section as a potential risk factor.

To examine the relationship between mode of first birth and the risk of subsequent preterm birth in New South Wales (NSW), Australia.

Materials and Methods
A population‐based record‐linkage study of NSW women who had a live singleton first birth at ≥37 weeks gestation, followed by a singleton second birth between 2005 and 2017. Relative risk (RR) and 95% CI of preterm birth in the subsequent pregnancy was calculated using modified Poisson regression, with mode of first birth as the exposure. Spontaneous preterm birth and preterm prelabour caesarean were secondary outcomes.

Women who had either an intrapartum (RR: 1.26, 95% CI 1.19–1.32) or prelabour caesarean (RR: 1.26, 95% CI 1.18–1.35) first birth had a higher risk of subsequent preterm birth (any birth <37 weeks gestation), than those who birthed vaginally. Women who had a previous instrumental birth (RR: 0.85, 95% CI 0.79–0.91) or prelabour caesarean (RR: 0.74, 95% CI 0.67–0.82) had lower risks of subsequent spontaneous preterm birth. However, prior prelabour caesarean also greatly increased risk of subsequent preterm prelabour caesarean (RR: 5.25, 95% CI 4.65–5.93). Conclusions The mode of first birth has differing effects on the risk of subsequent spontaneous preterm birth and preterm prelabour caesarean. Awareness of the risk of subsequent preterm birth following caesarean section may help inform clinical decisions around mode of first birth.