Objective: To assess possible effects of a cesarean delivery on outcome in subsequent pregnancies.
Methods: Using an historical cohort design, we analyzed 637,497 first and second births among women with two or more single births and 242,812 first, second, and third births among women with three or more single births registered in the population-based Medical Birth Registry of Norway between 1967 and 2003.
Results: Compared with a vaginal delivery at first birth, a cesarean delivery at first birth was followed, in a second pregnancy, by increased risks of preeclampsia (odds ratio [OR] 2.9 and corresponding 95% confidence interval [CI] 2.8-3.1), small for gestational age (OR 1.5; CI 1.4-1.5), placenta previa (OR 1.5; CI 1.3-1.8, placenta accreta (OR 1.9; CI 1.3-2.8), placental abruption (OR 2.0; CI 1.8-2.2), and uterine rupture (OR 37.4; CI 24.9-56.2). After excluding women with the actual complication at first birth, the corresponding ORs were, in general, lower: 1.7 (CI 1.6-1.8), 1.3 (CI 1.3-1.4), 1.4 (CI 1.2-1.7), 1.9 (CI 1.3-2.8), 1.7 (CI 1.6-1.9), and 37.2 (CI 24.7-55.9), respectively. Corresponding reduction in numbers of cesarean deliveries needed to prevent one case were 114, 56, 1,140, 3,706, 300, and 461. In third births, ORs after repeat cesarean delivery were similar to or lower than the ORs after one cesarean delivery; also here, the exclusion of women with the actual outcome in any of their previous pregnancies tended to reduce the ORs.
Conclusion: Cesarean delivery was associated with an increased risk of complications in a subsequent pregnancy, but excess risks were reduced after excluding women with the actual complication in any of their previous births. To obtain less biased effects of cesarean delivery on subsequent pregnancies, it is important to account for obstetric history.
Level of evidence: II.