Non-linear association between female body mass index and live birth after intrauterine insemination: a retrospective cohort study

Background : Evidence regarding the influence of female body mass index (BMI) on intrauterine insemination (IUI) outcomes remains inconsistent, potentially due to heterogeneous cutoff values, inclusion criteria, outcome definitions, or neglect of nonlinear relationships. This study aimed to evaluate the association between female BMI and live birth rate (LBR) following the first IUI cycle, with particular emphasis on characterizing potential nonlinear patterns. Methods : This retrospective cohort study included 5,651 women undergoing their first IUI cycle. Female BMI was analyzed both as a continuous and a categorical variable, with detailed exploration of nonlinear effects. The primary outcome was LBR; secondary outcomes included biochemical pregnancy, clinical pregnancy, biochemical pregnancy loss, cesarean delivery, ectopic pregnancy, miscarriage, and preterm birth. Multivariate logistic regression, smooth curve fitting, and threshold effect analysis were employed to determine the impact of BMI on LBR. Results : A nonlinear, inverted U-shaped relationship was identified between female BMI and LBR. The inflection point was 27.18 kg/m²(95% CI: 25.95–28.76). Below this threshold, each 1 kg/m²increment in BMI was associated with a 4% higher probability of live birth (OR 1.04, 95% CI: 1.01–1.08, P=0.005). Above the threshold, each additional BMI unit was associated with a 22% reduction in LBR (OR 0.78, 95% CI: 0.69–0.88, P<0.001). Sensitivity analyses restricted to women with a single ovulated follicle and stratified analyses by endometrial thickness tertiles confirmed the robustness of this nonlinear association. Similar patterns were observed for biochemical and clinical pregnancy rates. Women with BMI ≥ 28 kg/m²exhibited higher rates of biochemical pregnancy loss and cesarean delivery. No significant associations were found between Asian BMI categories and ectopic pregnancy, miscarriage, or preterm birth. Conclusion : The relationship between female BMI and LBR after the first IUI cycle presents an inverted U-shaped curve, with an optimal BMI around 27.18 kg/m². Both low and high BMI adversely affect live birth outcomes: a modest positive effect is seen in the non-obese range, whereas a pronounced negative effect emerges in the obese range.