Determinants of birth weight in boys and girls

Abstract

Boys are heavier at birth than girls, but girls have higher fat mass at birth than boys. Insulin action may play an important role of this different distribution. Animal models have shown that female offspring are more sensitive to maternal feeding and glucose values during pregnancy and weaning than male offspring. Newborn girls have higher insulin and proinsulin concentrations and total proinsulin-to-insulin ratios in cord blood than boys, despite lighter birth weight. In a cohort of 522 newborn above 37 gestational weeks we split between the sexes and studied associations between birth weights, parental anthropometrics and fasting maternal plasma glucose and insulin levels. Boys weighed 184g more than girls, they were 1.1 cm longer and head circumference differed by 0.86 (all p values <0.01). Multiple linear regressions showed that parity, maternal body mass index, gestational age and maternal birth weight were associated with birth weight for both sexes, whereas maternal weight gain in pregnancy and maternal fasting plasma glucose at week 30-32 were significantly associated with birth weight for girls only. The effect of fasting plasma glucose on birth weight in girls was twice as high as in boys (B=162, 95% CI 33.4-291, p=0.01). Paternal birth weight was significantly associated with birth weight of boys, but no such association was seen for girls. This supports the notion that there is a genetic regulation along the male line. Girls may be more sensitive to intrauterine environment and maternal glucose values, as these have a stronger influence on birth weight of girls.