Risk of Spontaneous Preterm Birth in a Subsequent Pregnancy After Full Dilatation Caesarean Birth: A Nationwide Cohort Study.

Objective: To investigate the association between caesarean birth in the second stage of labour and the risk of spontaneous preterm birth in a subsequent pregnancy.

Design: Nationwide register-based cohort study.

Setting: The Danish Medical Birth Registry and The Danish National Patient Register from 1997 to 2021.

Population: Women with at least two consecutive births (index birth and subsequent birth) where the index birth was a term singleton.

Methods: Women with spontaneous preterm subsequent birth were compared according to the mode of index birth, including vaginal, prelabour caesarean, first stage caesarean and second-stage caesarean. Statistical analysis was performed using multiple logistic regression.

Main outcome measure: Spontaneous birth before 37 + 0 gestational weeks.

Results: A total of 376 414 women met the inclusion criteria. Distribution of mode of index birth was vaginal 318 117 (84.5%), prelabour caesarean 15 373 (4.1%), first stage caesarean 37 547 (10.0%) and second-stage caesarean 5377 (1.4%), respectively. The rate of subsequent preterm birth was 1.7%. Compared to vaginal birth, a second-stage caesarean was associated with an increased risk of preterm birth in a subsequent pregnancy (adjusted odds ratio [aOR] 1.46, 95% confidence interval [CI] 1.21-1.77). Similarly, compared to first stage caesarean, a second-stage caesarean was associated with an increased risk of preterm birth in the subsequent pregnancy, with an aOR of 1.41 (95% CI 1.15-1.74).

Conclusion: Second-stage caesarean in a previous term pregnancy is associated with an increased risk of spontaneous preterm birth in a subsequent pregnancy.