First trimester maternal sex steroids and the digit ratios of mothers and children.

BACKGROUND: It is thought that digit ratios (2D:4D) are a correlate of 1st trimester maternal and foetal sex steroids. Here we consider the relationship of 2D:4D to the former. METHOD: Digit lengths were directly measured with a calliper at infant age 13 months. Measures of T and E were obtained from mother's blood at 6-8 weeks, 10-11 weeks and 1st trimester means were calculated. RESULTS: There were 69 mother-infant pairs (33 boys). Sex differences in 2D:4D (boysboys) were found. For mothers of girls: there were negative relationships between 2D:4D and T at 6-8 weeks, 10-11 weeks and 1st trimester means. For infants: girls showed more correlations between 2D:4D and hormones than boys. For boys, there was one positive association between 2D:4D and E and two positive associations for E/T. For girls, 2D:4D was negatively related to T (four correlations) and positively related to E/T (four correlations). Considering associations in the total sample and controlling for sex, at 6-8 weeks right and left 2D:4D were positively related to E. At 10-11 weeks, right and left 2D:4D were negatively related to T. For 1st trimester means, 2D:4D's were positively related to E (right and left) and negatively related to T (right). CONCLUSION: Infant 2D:4D was correlated with first trimester maternal sex steroids, particularly at 10-11 weeks. The correlations were negative for T, and positive for E and E/T with weaker effects for male infants. The latter pattern may arise because in boys T produced by foetal testes masks the effect of maternal T.