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.