Risk of preterm birth in the subsequent pregnancy following caesarean section at full cervical dilatation compared with mid-cavity instrumental delivery

Background: Expediting delivery in the second stage of labour often involves a choice between a caesarean section at full dilatation or mid-cavity instrumental delivery. Accumulating evidence suggests that the mode of delivery may influence the risk of preterm birth in the subsequent pregnancy. Aims: To directly compare first birth caesarean section at full dilatation with mid-cavity instrumental delivery for the risk of preterm birth in the subsequent pregnancy (second birth). A further aim was to identify predictive factors associated with these index modes of delivery. Materials and Methods: This is a retrospective cohort study involving three maternity hospitals in western Sydney over the period of 2006-2017. Inclusion criteria were nulliparous women with a singleton term cephalic first birth delivered by caesarean section at full dilatation or mid-cavity instrumental delivery, and whose second birth also occurred under our care. Data were analysed separately for first and second births. Results: There were 425 caesarean section at full dilatation and 874 mid-cavity instrumental cases which met inclusion criteria. The risk of preterm birth in the second birth was 5.7% compared to 3.2%, respectively (risk ratio 1.76; 95% CI 1.04-3.00; P=0.035). After excluding causes of preterm birth not related to previous mode of delivery, the risk of spontaneous preterm birth was 4.3% compared to 2.0%, respectively (risk ratio 2.18; 1.14-4.19; P=0.019). Conclusion: Caesarean section at full dilatation is associated with a significantly higher rate of preterm birth in the subsequent pregnancy compared to a mid-cavity instrumental delivery. This should be considered in second-stage mid-cavity decision-making.