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

Among women undergoing in vitro fertilization, underweight prepregnancy body mass indices may have limited impact on pregnancy and live-birth rates but are associated with increased preterm and low-birthweight delivery.

Volume 106, Issue 7, Pages 1742-1750


Jennifer F. Kawwass, M.D., Aniket D. Kulkarni, M.B.B.S., M.P.H., Heather S. Hipp, M.D., Sara Crawford, Ph.D., Dmitry M. Kissin, M.D., M.P.H., Denise J. Jamieson, M.D., M.P.H.



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


Retrospective cohort study using national data and log binomial regression.


Not applicable.


Women undergoing IVF in the United States from 2008 to 2013.



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.


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).


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.

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