Extremities of body mass index and their association with pregnancy outcomes in women undergoing in vitro fertilization in the United States.

OBJECTIVE: To investigate the associations among underweight body mass index (BMI), pregnancy, and obstetric outcomes among women using assisted reproductive technology (ART).

DESIGN: Retrospective cohort study using national data and log binomial regression.

SETTING: Not applicable.

PATIENT(S): Women undergoing IVF in the United States from 2008 to 2013.

INTERVENTION(S): None.

MAIN OUTCOME MEASURE(S): Pregnancy outcomes (intrauterine pregnancy, live birth rates) per transfer, miscarriage rate per pregnancy, and low birth weight and preterm delivery rates among singleton and twin pregnancies.

RESULT(S): For all fresh autologous in vitro fertilization (IVF) cycles in the United States from 2008 to 2013 (n = 494,097 cycles, n = 402,742 transfers, n = 180,855 pregnancies) reported to the national ART Surveillance System, compared with normal weight women, underweight women had a statistically significant decreased chance of intrauterine pregnancy (adjusted risk ratio [aRR] 0.97; 95% confidence interval [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; aRR 0.87; 95% CI, 0.86-0.88, respectively). Among cycles resulting in singleton pregnancy, both underweight and obese statuses were associated with increased risk of low birth weight (aRR 1.39; 95% CI, 1.25-1.54, aRR 1.26; 95% CI, 1.20-1.33, respectively) and preterm delivery (aRR 1.12; 95% CI, 1.01-1.23, aRR 1.42; 95% CI, 1.36-1.48, respectively). The association between underweight status and miscarriage was not statistically significant (aRR 1.04; 95% CI, 0.98-1.11). In contrast, obesity was associated with a statistically significantly increased miscarriage risk (aRR 1.23; 95% CI, 1.20-1.26).

CONCLUSION(S): Among women undergoing IVF, prepregnancy BMI affects pregnancy and obstetric outcomes. Underweight status may have a limited impact on pregnancy and live-birth rates, but it is associated with increased preterm and low-birth-weight delivery risk. Obesity negatively impacts all ART and obstetric outcomes investigated.