Non-Linear Association Between Maternal Body Mass Index and Clinical Outcomes in Single Euploid Embryo Transfer: A...

To investigate the associations between maternal body mass index (BMI) and pregnancy outcomes in single euploid frozen embryo transfer cycles.

This single-center retrospective study analysed 4235 single euploid blastocyst transfer (SEBT) cycles performed between January 2018 and December 2024. Clinical outcomes were analysed using generalized estimating equations to assess the impact of BMI categories (underweight, normal, overweight, obese). Restricted cubic splines and Random forest models were employed to characterize the association between continuous BMI and clinical outcomes.

Clinical pregnancy rates were significantly lower in underweight (34.6%; aOR 0.66, p = 0.001) and obese women (37.5%; aOR 0.69, p = 0.014) than in normal-weight women (44.5%), whereas no significant difference was observed in the overweight group. Obesity was associated with increased odds of clinical miscarriage (aOR 2.17, p = 0.006) and preterm birth (aOR 3.05, p = 0.002), while underweight status had no significant effect on either outcome. Accordingly, live birth rates were reduced across all abnormal BMI categories, with the strongest effect observed in obese women (aOR 0.58, p < 0.001), followed by underweight (aOR 0.69, p = 0.006) and overweight women (aOR 0.85, p = 0.048). GEE-based restricted cubic spline analyses showed significant inverted U-shaped associations of maternal BMI with clinical pregnancy and live birth, with peak odds at BMI 21.9 kg/m² and 21.5 kg/m², respectively (clinical pregnancy: p for trend = 0.001, p for non-linearity < 0.001; live birth: p for trend < 0.001, p for non-linearity = 0.005). A similar single-peaked pattern was observed in the random forest models and remained broadly consistent across age groups and blastocyst-quality strata.

Maternal BMI exerts a stage-specific, non-linear influence on reproductive outcomes in euploid embryo transfers. Underweight status predominantly hinders pregnancy establishment, whereas obesity exerts a broader adverse effect across both pregnancy establishment and subsequent continuation. These findings support individualized, stage-informed counselling and highlight preconception weight optimization as a clinically actionable target to improve overall treatment success.