The associations of maternal body mass index with birthweight and placental weight. Does maternal diabetes matter? A population study of 106 191 pregnancies.

INTRODUCTION: The aim of this study was to study whether the associations of maternal body mass index with offspring birthweight and placental weight differ by maternal diabetes status. MATERIAL AND METHODS: We performed a population study of 106 191 singleton pregnancies by using data from the years 2009-2012 in the Medical Birth Registry of Norway. We estimated changes in birthweight and in placental weight (in grams) by maternal body mass index by linear regression analysis. RESULTS: In pregnancies of women without diabetes, birthweight increased by 14.7 g (95% confidence interval 14.1-15.2) per unit increase in maternal body mass index, and the increase in placental weight was 4.2 g (95% confidence interval 4.0-4.4). In pregnancies of women with gestational diabetes, the corresponding figures were 11.8 g (95% confidence interval 8.3-15.4) and 2.9 g (95% confidence interval 1.7-4.0). In pregnancies of women with type 1 diabetes we found no significant changes in birthweight or in placental weight by maternal body mass index. Overall, mean birthweight was 513.9 g (95% confidence interval 475.6-552.1) higher in pregnancies involving type 1 diabetes than in pregnancies of women without diabetes. Mean placental weight was 102.1 g (95% confidence interval 89.3-114.9) higher. Also, in pregnancies of women with gestational diabetes, both birthweight and placental weight were higher than in women without diabetes (168.2 g and 46.5 g, respectively). Adjustments were made for maternal body mass index and gestational age at birth. CONCLUSIONS: Birthweight and placental weight increased with increasing maternal body mass index in pregnancies of women without diabetes and in pregnancies of women with gestational diabetes, but not in pregnancies of women with type 1 diabetes. Independent of body mass index, mean birthweight and mean placental weight were highest in pregnancies of women with type 1 diabetes.