Body mass index and reported contraceptive use prior to the index pregnancy among Chinese women with unintended pregnancy: a multicenter cross-sectional analysis of a prospective cohort

Abstract Background Unintended pregnancy remains a major public health challenge globally and in China. While body mass index (BMI) is associated with adverse obstetric and metabolic outcomes, its relationship with contraceptive use among women with unintended pregnancy remains poorly characterized in large Chinese populations. This study investigated the cross-sectional association between BMI and reported contraceptive use prior to index pregnancy, and explored its clinical and public health implications. Methods This cross-sectional analysis utilized baseline data from a multicenter prospective cohort study conducted at 10 hospitals across 8 Chinese provinces and municipalities (May 2019–February 2022). A total of 8,125 women with unintended pregnancy were included. Multivariable logistic regression models were applied with BMI as both continuous and categorical variables (underweight, normal weight reference, overweight, obese). Among contraceptive users, we further examined the association between BMI and the use of highly effective (HEC) versus less effective (LEC) contraception. Sensitivity analyses using random forest imputation and complete-case analysis were performed. Results After full adjustment, each 1-unit increase in BMI was associated with 2% lower odds of reported contraceptive use (OR = 0.98, 95% CI: 0.96–0.99; P = 0.003). Women with overweight (OR = 0.85, 95% CI: 0.74–0.98; P = 0.024) and obesity (OR = 0.75, 95% CI: 0.57–0.98; P = 0.035) had significantly lower odds of contraceptive use compared to women with normal weight, whereas no significant association was observed for women with underweight. Among contraceptive users, BMI showed no significant association with the use of highly effective contraception (HEC) versus less effective contraception (LEC), regardless of whether BMI was modeled as a continuous or categorical variable. These findings remained robust in sensitivity analyses. Conclusions Overweight and obesity are independently associated with lower odds of contraceptive use among Chinese women with unintended pregnancy. These findings highlight the need for targeted contraceptive counseling for women with elevated BMI to reduce the burden of unintended pregnancy.