Abstract: Objective: To evaluate the effects of low body mass index (BMI) on in vitro fertilization-embryo transfer (IVF-ET) outcomes in fresh transfer cycles. Methods：A total of 8 171 cycles with conventional stimulation and fresh transfer in our IVF/ＩＣＳＩ center from January 2013 to October 2018 were included. Patients with BMI<18.5kg/m2 were considered to be underweight (n=774), and patients with normal BMI (18.5-23.9 kg/m2) were used as the control (n=7 397). Results：The biochemical pregnancy rate and implantation rate of the low BMI group were significantly lower than those of the control group (P＜0.05). There were no significant differences in the clinical pregnancy rate, early abortion rate and live birth rate between the two groups(P＞0.05). Conclusions：Low BMI is a negative factor of ART fresh transfer cycles.

Key words: Low body weight；, Body mass index；, Fertilization in vitro；, Embryo transfer；, Pregnancy rate；Live birth