Objective: To estimate the risk of preterm birth associated with mode of delivery in a previous pregnancy.

Patients and Methods: This was a multicenter retrospective cohort study. Women who had two consecutive deliveries in the participating hospitals between January 2012 and January 2022 were included in this study. They were divided into three groups according to the mode of delivery in a previous pregnancy: vaginal birth (group A), cesarean section before the second stage of labor (group B), and second stage cesarean section (group C). Data were extracted from the medical records. The primary outcome was the risk of spontaneous preterm birth in the subsequent pregnancy. The secondary outcomes were the risk of early spontaneous preterm birth (< 34 weeks) and the medically indicated preterm birth in the subsequent pregnancy. The logistic regression was employed to estimate odds and adjust for confounders.

Results: 18,253 women were included. Among them, 10,951 women were in group A, 5111 women in group B, and 2191 women in group C. The rates of spontaneous preterm birth in groups A, B, and C were 5.5%, 7.3%, and 15.1%, respectively. The rates of early spontaneous preterm birth in groups A, B, and C were 2.3%, 3.4%, and 8.2%, respectively. When compared with vaginal birth, second-stage cesarean section significantly increased the risk of spontaneous preterm birth (aOR, 3.23; 95% CI, 2.02–5.17; P = 0.003) and early spontaneous preterm birth (aOR, 3.59; 95% CI, 2.01–5.19; P <0.001) in the subsequent pregnancy. The rates of medically indicated preterm birth in groups A, B, and C were 2.3%, 2.8%, and 2.2%, respectively. There was no statistical difference across the three groups.

Conclusion: History of cesarean section at full dilation is independently associated with an increased risk of spontaneous preterm birth in the subsequent pregnancy.