Non-linear and context-dependent association of maternal BMI with cumulative live birth in Chinese women undergoing intrauterine insemination: a retrospective study of 3788 cycles

Background Evidence regarding maternal body mass index (BMI) and intrauterine insemination (IUI) outcomes remains controversial. This study aimed to evaluate such independent associations, focusing on potential non-linear patterns, clinical thresholds, and population-specific heterogeneities. Methods Data from 1951 couples (3788 cycles) were retrospectively analyzed. Multivariable generalized linear models (GLM), generalized estimating equations (GEE), and Cox proportional hazards models assessed first-cycle, per-cycle, and cumulative success, respectively; generalized additive model (GAM) and two-piecewise linear regression characterized non-linear patterns and thresholds. Results While maternal BMI showed no significant independent association with clinical pregnancy or live birth in the first cycle (all P > 0.05), per-cycle analysis of 3788 cycles revealed a modest positive correlation (pregnancy: aOR 1.04, P = 0.004; live birth: aOR 1.03, P = 0.030). Notably, cumulative success followed a non-linear pattern ( P LRT = 0.027), with live birth probability increasing until a BMI of approximately 21.2 kg/m² (aHR: 1.12, P = 0.007) but plateauing thereafter, potentially linked in part to an exploratory observation of higher spontaneous abortion rates in the obesity group (29.03%, P = 0.087). Subgroup analyses suggested potential heterogeneities in these associations across basal FSH levels and treatment protocols used in the first cycle ( P interaction = 0.030 and 0.009, respectively). Specifically, for the FSH < 8mIU/mL group, a non-linear association was suggested ( P LRT = 0.020), with success increasing up to approximately 21.2 kg/m² (aHR: 1.12, P = 0.011) and plateauing thereafter. Similarly, for patients whose first cycle used a gonadotropin protocol, a potential reversal was observed beyond approximately 21.0 kg/m² ( P LRT = 0.020), where the trend shifted to a significant decline (aHR: 0.81, P = 0.023). Conclusions Maternal BMI appears to exhibit a non-linear, context-dependent association with cumulative IUI success, underscoring the potential need for individualized preconception management.