Objective:

To identify an exploratory BMI threshold associated with increased risk of macrosomia and other adverse neonatal outcomes after fresh embryo transfer (fresh ET).

Approach:

• Study Design: Retrospective cohort study including 2,195 women who underwent autologous fresh ET between June 2019 and December 2023.
• Data Analysis: Multivariable regression analysis examined the association between prepregnancy BMI and neonatal outcomes; smooth curve fitting and threshold effect analysis identified the exploratory BMI threshold.
• Sensitivity Analysis: Adjusted analysis following propensity score matching (PSM) was conducted to validate the robustness of the results.

Key Findings:

• Birthweight and Z-score were positively associated with increasing maternal BMI (adjusted β: 22.63, 95% CI: 14.53 to 30.73; P<0.001).
• The incidence of macrosomia increased by 2.25-fold in the BMI 25-29.9 kg/m² group (adjusted OR: 2.25, 95% CI: 1.29 to 3.93; P = 0.004) and by 4.56-fold in the BMI ≥30 kg/m² group (adjusted OR: 4.56, 95% CI: 2.26 to 9.22; P<0.001) compared to the reference group (BMI 18.5-24.9 kg/m²).
• A significant increase in the odds of macrosomia was observed when BMI exceeded 26.22 kg/m² (adjusted OR: 4.05, 95% CI: 2.47 to 7.29; P = 0.0009).

Interpretation:

A prepregnancy BMI exceeding approximately 26 kg/m² is associated with an increased risk of macrosomia in singleton pregnancies conceived via fresh ET.

Limitations:

• The study is retrospective and may be subject to biases.
• External validation of the identified BMI threshold is needed.

Conclusion:

The exploratory threshold of BMI >26.22 kg/m² may provide insights for further research on risk communication and weight management in women undergoing IVF treatment.