Assisted reproductive technology cycle and obstetric outcomes among underweight and overweight women

To investigate the association between body mass index (BMI) and in vitro fertilization (IVF) and pregnancy outcomes among women using Assisted Reproductive Technology (ART). Retrospective cohort study using the National ART Surveillance System (NASS). We assessed BMI trends among women undergoing fresh autologous IVF in the US from 2008-2013 (n=494,097 cycles) using the NASS. We calculated pregnancy outcomes (intrauterine pregnancy, live birth rates) among non-cancelled transfers (n=402,742) in underweight (BMI <18.5 kg/m2), normal weight (BMI 18.5-24.9 kg/m2), and obese women (BMI >30kg/m2). Among cycles resulting in pregnancy (n=180,855), we calculated miscarriage rate. We calculated low birth weight (<2500g) and preterm (<37 weeks) delivery rates among singleton and twin pregnancies separately. We used log-binomial regression to investigate the relationship between BMI and pregnancy outcomes. Compared with normal weight women, underweight women had a statistically decreased but clinically comparable chance of intrauterine pregnancy (aRR=0.97,95% CI=0.96-0.99) and live birth (aRR=0.95,95% CI=0.93-0.98) per transfer; obese women also had a statistically decreased likelihood of both (aRR=0.94, 95% CI=0.94-0.95 and aRR 0.87, 95% CI=0.86-0.88, respectively). The association between underweight BMI and miscarriage was not statistically significant (aRR=1.04, 95% CI=0.98-1.11). In contrast, obesity was associated with a significantly increased miscarriage risk (aRR=1.23, 95% CI=1.20-1.26). Among cycles resulting in singleton pregnancy, both underweight and obese statuses were associated with increased risk of low birth weight (aRR=1.33, 95% CI=1.18-1.49, aRR=1.26, 95% CI=1.19-1.33) and preterm delivery (aRR=1.12, 95% CI=1.01-1.23, aRR=1.42, 95% CI=1.36-1.48). Results were similar in twin pregnancies. Among women undergoing IVF, pre-pregnancy BMI affects pregnancy outcomes. Although underweight status may have limited impact on pregnancy and live birth rates, it is associated with increased preterm and low birth weight delivery risk. Obesity negatively affects all pregnancy outcomes investigated.