Lack of effect of maternal body mass index on anthropometric characteristics of newborns in twin gestations

We examined the correlation between maternal prepregnancy body mass index (BMI) and newborn weight, length, BMI, and gestational order, in singleton and twin births. The sample comprised 381 mothers of multiple babies (562 twins), and 7979 singleton pregnancies, used as controls. The Mann-Whitney non-parametric test was used to compare the values between the two groups, and the Spearman's correlation test (rS) was applied to the quantitative variables. A significant positive correlation was found with singleton baby variables: the higher the maternal BMI, the higher the newborn's BMI, weight, length, and gestational order. However, no significant correlation was found between maternal BMI and any of these variables in twins. Maternal weight gain, in the twin group, showed a significant positive correlation with the newborn gestational order (rS = 0.154; P = 0.002), weight (rS = 0.493; P < 0.001), length (rS = 0.469; P < 0.001), and BMI (rS = 0.418; P < 0.001). In singletons, the correlation was positive with all the variables, except for the gestational order. The newborn BMI was significantly higher in twins born by C-section than those born by vaginal birth (Z = -4.974; P < 0.001). Mothers of singletons delivered by C-section had a significantly higher BMI than those of singletons born by vaginal birth (Z = -1.642; P < 0.001); however, no significant differences were observed in mothers of twins. Prepregnancy maternal BMI in twin births would not be predictive of newborns weight, length and BMI in this population. Maternal weight gain during pregnancy proved to be the most adequate for predicting the weight, length and BMI of twins delivered by C-section.