Maternal characteristics (Table 1)
In the entire cohort, the mean maternal age was 33.3 years (SD 2.9), and women with an IPI of 120 months or greater were significantly older than the other three groups, with a value of 37.1 years (SD 3.0). To test the collinearity of the interpregnancy interval and maternal age, we performed a collinearity diagnostic, which indicated low collinearity between the two factors. Women with an IPI of 120 months or greater had a higher prevalence of gravidity ≥ 3 than the other four groups while having a lower prevalence of parity ≥ 2 and prior CS ≥ 2 and a smaller gestational age at delivery. In addition, significant differences were found among the four groups in terms of the distributions of maternal body mass index (BMI) (p = 0.005) and educational status (p < 0.001). Women with an IPI of less than 24 months had a history of 2 CSs or more, and among them, the proportion (3.5%) of in vitro fertilization and embryo transfer (IVF-ET) was higher than the rates among the other three groups. There were no significant differences in the distribution of history of vaginal delivery or in the proportion of pregnant women with chronic hypertension or pregestational diabetes mellitus (PGDM) among the four groups. Only 2.8% (n = 47) of women achieved VBAC, and the rates of emergency CS and CS with indications were not different across the four groups.