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808: The relationship between maternal body mass index and pregnancy outcomes in twin pregnancies

      Objective

      High maternal body mass index (BMI) is associated with pregnancy complications in singleton pregnancies including preeclampsia, gestational diabetes, preterm birth, and caesarean section. The effects of maternal BMI on pregnancy outcomes in women with twins, who are a priori at an increased risk for these complications, are unclear. Furthermore, given the increased nutritional demands in twin pregnancies, the adverse effects of low maternal BMI may be greater in twins. Our aim was to assess the associations between maternal BMI and adverse outcomes in twin pregnancies and to compare them to those observed in singletons.

      Study Design

      The study group included all women with a twin or singleton pregnancy who gave birth in Ontario, Canada between 2012-2016. Data were obtained from the Better Outcomes Registry & Network (BORN) Ontario database. The associations between maternal BMI group and pregnancy complications were compared between twin and singleton pregnancies.

      Results

      1) Out of 492,151 women who met the study inclusion criteria, 484,184 (98.4%) had a singleton pregnancy and 7,937 (1.6%) had twins. 2) The adjusted relative risk of preeclampsia and gestational diabetes increased with maternal BMI in both twins and singletons, although these associations were stronger in singletons than in twins (Figure 1). 3) The associations between maternal BMI and birth weight >90% or <10th% for gestational age were similar in both twins and singletons (Figure 1). 4) While the risk of preterm birth in singletons increased with maternal BMI, the risk of preterm birth in twins was highest for underweight women, and was not affected by high maternal BMI (Figure 2). 5) The risk of NICU admission and low Apgar score in singletons increased with maternal BMI, but such an association was not observed in twins.