Main findings
In this retrospective cohort study of 821 women undergoing IVF with euploid blastocyst embryo transfer, we showed that live birth was not significantly different between obese and normal-weight women.
The prevalence of obesity continues to rise worldwide. The negative consequences of obesity are identifiable in every organ system. Obesity is a known risk factor for subfertility, miscarriage, feto-maternal complications and long-term risks in adult life [9]. Obese women are at particularly high risk for adverse pregnancy outcomes. Up till now, the relationship between obesity and negative reproductive outcomes is clear, but the mechanism by which obesity affects fertility remains unclear. Data from large trials have demonstrated fewer normally fertilized oocytes, lower pregnancy and live birth rates in obese women [10]. We believe that our study adds important information to the existing literature on pre-pregnancy BMI. A large cohort study of 152,500 cycles similarly reported significantly higher odds of cycle cancellation, pregnancy failures with overweight women [11]. A systematic review and meta-analysis found decreased pregnancy rates, increased gonadotropins and a higher miscarriage rate in obese and overweight women. These differences are evidence even at a BMI≥ 25 kg/m2[12].
Our study differs from previous studies on obesity and clinical outcomes in three ways. First, most studies dealt with embryo transfer without PGT-A. Theorizing that oocytes from obese patients are subject to alterations in normal mitotic checkpoints leading to aneuploidy [13]. Therefore, they cannot accurately elucidate the effects of BMI on clinical outcomes in obese women. Second, many studies dealt with obesity as a categorical variable. In contrast, in our study, BMI was also analyzed as a continuous variable that allowed a subtle increase in live birth. Third, some studies compared clinical outcomes in fresh cycles. One possible mechanism suggested that endometrial advancement induced by ovarian hyperstimulation in a fresh embryo transfer cycle, resulting in embryo-endometrium asynchrony. Therefore, our study focused on frozen embryo transfer cycles, which eliminate the negative effect of controlled ovarian stimulation.