Effects of body mass index on pregnancy outcomes of frozen-thawed embryo transfer in different age groups

This study aimed to explore the effects of body mass index on the pregnancy outcomes in different age groups in women undergoing their first frozen-thawed embryo transfer cycle. A total of 1006 patients undergoing their first single high-quality blastocyst FET between January 2019 and January 2023 at the Center of Reproductive Medicine, the Fourth Hospital of Shijiazhuang, were considered in this retrospective cohort study. First, the included patients were classified into three groups according to their BMI: underweight (BMI < 18.5 kg/m2, 62 patients), normal weight (18.5 ≤ BMI < 24 kg/m2, 526 patients), overweight/obese (BMI ≥ 24 kg/m2, 418 patients). Subsequently, participants were further stratified by age into three subgroups: < 30 years, 30–34 years, and ≥ 35 years. Clinical pregnancy rate, live birth rate, and miscarriage rate were compared across BMI categories and within each age subgroup. Multivariate logistic regression analyzed independent effects and Age × BMI interaction. When stratified by BMI alone, there were no statistically significant differences in pregnancy outcomes across groups (P > 0.05). Age-stratified analysis with Bonferroni correction showed that among women aged < 30 years and ≥ 35 years, no significant differences were observed in clinical pregnancy rate, live birth rate, or miscarriage rate across BMI subgroups (P > 0.05). However, within the 30–34 years age group, the miscarriage rate in the overweight/obese group was significantly higher than that in the normal weight group, with a statistically significant difference (P = 0.005). No significant Age × BMI interaction was observed (all P > 0.05). Multivariate binary logistic regression analysis showed that advanced age was an independent risk factor for both clinical pregnancy and live birth rate in frozen-thawed embryo transfer (FET) cycles, whereas higher BMI was an independent risk factor for miscarriage. Although the BMI can greatly affect the miscarriage rate of women aged 30–34 years, it has almost no effects on the outcomes of younger or older women. Advanced age and higher BMI are independent risk factors for adverse outcomes.