First trimester maternal sex steroids and head circumference in newborns

There is indirect evidence (from digit ratio [2D:4D] research) that prenatal oestrogen is positively related to neonate head circumference (HC), with stronger effects in males. Here we test this theory directly by considering the relationship between maternal first trimester sex steroids (oestradiol [E] and testosterone [T]) and the HC of neonates.

Measures of E and T were obtained from mother's blood at 6-8 weeks (E1, T1), and 10-11 weeks (E2, T2). Neonate HC, length, and weight were recorded together with maternal anthropometrics.

There were 47 neonates (24 boys) and their mothers. Mothers with girls had higher values of E1, T1 and E2 than mothers with boys. There were no mother‑neonate sex differences for age, height, weight, BMI, and weight gain during pregnancy. Neonates showed no sex differences for HC, length or birthweight. HC was negatively related to age at pregnancy and positively related to E1. There were no other univariate correlations with HC. Multiple regression with HC as dependent variable showed a positive relationship with E1 and male sex and no effects for maternal age, T1, E2 or T2. Splitting by sex showed positive correlations between HC and male or female E1 with the former stronger than the latter.

HC was positively correlated with maternal E1, independent of T1, E2, T2 and maternal anthropometrics. Splitting by sex, the relationship between HC and E1 was stronger for male neonates compared to female neonates. Our direct findings support earlier reports of positive correlations between prenatal E (which were indirectly measured by 2D:4D) and HC, and that these effects are stronger for boys than girls.