Reproductive and obstetric outcomes in mildly and significantly underweight women undergoing IVF

Research question: What is the impact of low body mass index (BMI) on live birth rates and obstetric outcomes in infertile women treated with IVF and fresh embryo transfer? Design: This was a retrospective cohort study of infertile patients in an academic hospital setting who underwent their first oocyte retrieval with planned autologous fresh embryo transfer between 1 January 2012 and 31 December 2018. The primary study outcome was live birth rate. Secondary outcomes were IVF treatment and delivery outcomes. Underweight patients were stratified into a significantly underweight group (body mass index [BMI] <17.5 kg/m(2)) and a mildly underweight group (BMI 17.5-18.49 kg/m(2)), and were compared with a normal-weight group (BMI 18.5-24.9 kg/m(2)). Results: A total of 5229 patients were included (significantly underweight, 76; mildly underweight, 231; normal weight, 4922), resulting in 4798 embryo transfers. After oocyte retrieval, there were no significant differences between groups for total oocytes, mature oocyte yield and number of supernumerary blastocysts cryopreserved. Among women who had an embryo transfer, there were no significant differences in the live birth rates in significantly (31.0%, odds ratio [OR] 0.67, confidence interval [0.95, CI] 0.40-1.13) and mildly (37.7%, OR 0.95, CI 0.73-1.33) underweight patients compared with normal-weight patients (35.9%). Additionally, there were no statistically significant increased risks of preterm delivery, Caesarean delivery or a low birthweight (<2500 g) neonate. Conclusions: Mildly and significantly underweight infertile women have similar pregnancy and live birth rates to normal-weight patients after IVF treatment. In addition, underweight patients do not have an increased risk of preterm delivery (<37 weeks), Caesarean delivery or a low birthweight neonate.