Introduction
According to the recommendations of the World Health Organization (WHO), the interval from previous live birth and subsequent conception should be a minimum of 2 years, regardless of the previous delivery mode.1 In 2016, when the universal one-child policy was replaced with the two-child policy, all couples in China were allowed to have a second child.2 Because the one-child policy had been in place for 36 years in China, this change was very likely to increase the number of high-risk parturients with typical risk factors such as advanced maternal age (AMA) and a long interpregnancy interval (IPI). Additionally, there has also been a sharp increase in pregnancies with previous cesarean section (CS) due to the high CS rates over recent decades in China.3,4 Consequently, it is a great challenge for obstetricians in China to manage the growing number of high-risk parturients with multiple risk factors, such as AMA, a long IPI, and previous CS, which have been demonstrated to increase the risk of adverse pregnancy outcomes, such as uterine rupture,5,6 gestational hypertension (GHP),7,8 gestational diabetes mellitus (GDM),9 preterm delivery, low birth weight (LBW) and neonatal intensive care unit (NICU) admission.10-15
We performed a preliminary systematic review of the association between IPI and adverse pregnancy outcomes after previous CS and found that pregnancy spacing after previous CS was related to the risk of uterine rupture, the probability of a successful vaginal birth after CS (VBAC), and the risk of placenta previa.16 However, we cannot conclude the optimal IPI after previous CS because different measurements and categorizations of birth spacing were used in previous studies. Furthermore, 36 years after implementing the one-child policy, China implemented the two-child policy broadly; as a result, the number of pregnant women with long IPIs increased significantly, and this family planning policy caused the number of pregnancies and deliveries among Chinese women to be quite different from those of other countries. Thus, there is no relevant research conducted exclusively for Chinese women. Therefore, we conducted this large retrospective cohort study to explore the association between IPI after previous CS and adverse pregnancy outcomes.