NB female's digit ratio, but not males', was associated to the level of AF testosterone. The lower 2D:4D ratios in mothers who had sons support claims that hormone levels in parents are influential for determining their children's sex. Sexual dimorphism at birth was only significant for the left hand, in contrast with reports of greater right hand dimorphism, suggesting that postnatal testosterone is determinant for 2D:4D stabilization. Maternal plasma testosterone also showed a negative weak correlation with NB's digit ratio in both sexes. Both hands of NB females were negatively correlated with AF testosterone and positively correlated with the mother's 2D:4D, but males showed no significant associations. Mothers who had sons had lower 2D:4D ratios than those who had daughters and the mother's 2D:4D were higher than those of NBs regardless of sex. NB males had lower mean 2D:4D ratios than females but this dimorphism was significant only for the left hand (males: 0.927 females: 0.950 p = 0.004). Shortly after birth, 106 NBs and their mothers were measured for 2D:4D ratio. Testosterone was assayed in samples of maternal plasma and AF collected at amniocentesis. We characterize the 2D:4D ratio in newborn (NB) infants, in between the pre- and postnatal surges of testosterone, and relate it to the mother's 2D:4D and to testosterone levels in the amniotic fluid (AF). The 2D:4D digit ratio is sexually-dimorphic, probably due to testosterone action through the perinatal period.
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