Pre-pregnancy obesity is not associated with poor outcomes in fresh transfer in vitro fertilization cycles: a retrospective study

PURPOSE: The impact of body mass index (BMI) on in vitro fertilization (IVF) has been well acknowledged; however, the reported conclusions are still incongruent. This study aimed to investigate the effect of BMI on IVF embryos and fresh transfer clinical outcomes. METHODS: This retrospective cohort analysis included patients who underwent IVF/ICSI treatment and fresh embryo transfer from 2014 to March 2022. Patients were divided into the underweight group: BMI < 18.5 kg/m(2); normal group: 18.5 ≤ BMI < 24 kg/m(2); overweight group: 24 ≤ BMI < 28 kg/m(2); and obesity group: BMI ≥ 28 kg/m(2). A generalized linear model was used to analyze the impact of BMI on each IVF outcome used as a continuous variable. RESULTS: A total of 3465 IVF/ICSI cycles in the embryo part; and 1698 fresh embryo transplanted cycles from the clinical part were included. Available embryos rate (61.59% vs. 57.32%, p = 0.007) and blastocyst development rates (77.98% vs. 66.27%, p < 0.001) were higher in the obesity group compared to the normal BMI group. Also, the fertilization rate of IVF cycles in the obesity group was significantly decreased vs. normal BMI group (normal: 62.95% vs. 66.63% p = 0.006; abnormal: 5.43% vs. 7.04%, p = 0.037), while there was no difference in ICSI cycles. The clinical outcomes of overweight and obesity groups were comparable to the normal group. The gestational age of the obesity group was lower compared to the normal group (38.08 ± 1.95 vs. 38.95 ± 1.55, p = 0.011). The adjusted OR (AOR) of BMI for the preterm birth rate of singletons was 1.134 [(95% CI 1.037–1.240), p = 0.006]. BMI was significantly associated with live birth rate after excluded the PCOS patients [AOR: 1.042 (95% CI 1.007–1.078), p = 0.018]. In young age (≤ 35 years), clinical pregnancy rate and live birth rate were positively correlated with BMI, AOR was 1.038 [95% CI (1.001–1.076), p = 0.045] and 1.037 [95% CI (1.002–1.074) p = 0.038] respectively. CONCLUSION: Being overweight and obese was not associated with poor IVF outcomes but could affect blastocyst formation. ICSI could help to avoid low fertilization in obese patients. Also, obesity was associated with increased rates of premature singleton births. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12884-023-05917-7.