Weight gain and pregnancy outcomes in underweight women with twin gestations

Background: Adherence to minimum weight gain recommendations in twin pregnancies as defined by the Institute of Medicine (IOM) guidelines is associated with improved pregnancy outcomes; however, these recommendations have yet to be made for underweight women (BMI < 18.5 kg/m²). Objective: To determine if existing pregnancy weight gain recommendations in twin gestations apply to women with underweight prepregnancy body mass index (BMI), and if these women have similar pregnancy outcomes to women with normal prepregnancy BMI. Methods: This is a retrospective cohort study of women with underweight (BMI < 18.5 kg/m²) and normal prepregnancy BMI (18.5–24.9 kg/m²) and twin pregnancies delivered > 24 weeks by a single Maternal Fetal Medicine (MFM) practice between 2005–2017. Weight gain patterns and pregnancy outcomes were compared between women with underweight prepregnancy BMI and normal prepregnancy BMI. The primary outcomes of the study were gestational age at delivery and intrauterine growth restriction. Results: A total of 664 women met inclusion criteria during the study period, 55 (8.3%) of whom had an underweight prepregnancy BMI, and 609 (91.7%) of whom had a normal prepregnancy BMI. There were no differences in baseline clinical and demographic characteristics between the groups, nor any differences in overall weight gain or weight gain patterns between the two groups. There were no significant differences between the groups in birthweight of the larger twin (2415 versus 2489 g, p =.26) or the smaller twin (2150 versus 2190 g, p =.55), gestational age at delivery (35.8 versus 35.8 weeks, p =.96), incidence of preterm birth < 34 weeks (16.4 versus 16.3%, p =.98), spontaneous preterm birth < 34 weeks (9.1 versus 11.7%, p =.57), or the incidence of either twin with a birthweight < 10% for gestational age (60.0 versus 56.0%, p =.57) or < 5% for gestational age (36.4 versus 30.9%, p =.40). Conclusions: Women with underweight prepregnancy BMI who gain equal weight to women with normal prepregnancy BMI have similar pregnancy outcomes. The recommendations for adequate weight gain in women with underweight prepregnancy BMI should therefore not exceed those for women with normal prepregnancy BMI.