Caesarean birth and risk of subsequent preterm birth: a retrospective cohort study.

OBJECTIVE: To determine the risk of spontaneous and medically indicated preterm birth associated with mode of birth in previous term-born pregnancy. DESIGN: Retrospective cohort study. SETTING: Two UK maternity units. POPULATION OR SAMPLE: A total of 16 340 women with first two consecutive singleton births and the first birth at term. METHODS: Retrospective cohort study using routinely collected clinical data. MAIN OUTCOME MEASURES: Incidence of spontaneous preterm birth and medically indicated preterm birth at less than 37 weeks of gestation after term birth, in relation to mode of birth in first pregnancy. Subgroup analysis on cervical dilatation at the time of first caesarean birth. RESULTS: Compared with vaginal birth, emergency caesarean birth at full dilatation was associated with an increase in spontaneous preterm birth (2.3% vaginal birth versus 4.5% full dilatation caesarean; adjusted odds ratio [aOR] 3.29, 95% CI 2.02-5.13, P < 0.001). Elective caesarean, emergency caesarean at <4 cm dilatation, and emergency caesarean at 4-9 cm dilatation were associated with increased medically indicated preterm birth (0.8% vaginal births versus 1.9% elective caesarean, 3.3% <4 cm caesarean, 1.3% 4-9 cm caesarean; aOR 2.30, 95% CI 1.19-4.15, P = 0.009; aOR 4.68, 95% CI 2.98-7.24, P < 0.001; and aOR 2.43, 95% CI 1.43-4.00, P = 0.001, respectively). CONCLUSIONS: Term caesarean in the first stage of labour or performed prelabour is associated with medically indicated preterm birth. Term caesarean in the second stage of labour is associated with spontaneous preterm birth. TWEETABLE ABSTRACT: Caesarean in the second stage of labour is associated with spontaneous preterm birth.