Introduction: The 2D:4D digit ratio is sexually-dimorphic, probably due to testosterone action through the perinatal period. 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). Subjects and methods: Testosterone was assayed in samples of maternal plasma and AF collected at amniocentesis. Shortly after birth, 106 NBs and their mothers were measured for 2D:4D ratio. Results: 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). 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. 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. Maternal plasma testosterone also showed a negative weak correlation with NB's digit ratio in both sexes. Conclusions: 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. 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. NB female's digit ratio, but not males', was associated to the level of AF testosterone. The mother's 2D:4D ratios were positively correlated with their daughters' 2D:4D, but the same was not observed for male NBs, suggesting that prenatal testosterone levels in male fetus lead their 2D:4D ratios to stray from their mothers' with high individual variability.
Foetal sex hormones can have powerful and far-reaching effects on later phenotype. However, obtaining accurate measurements is difficult for ethical reasons, and researchers often employ proxy variables to examine their effects. The relative length of the second and fourth fingers (digit ratio or 2D:4D) is frequently used for this purpose, as it is hypothesized to index variance in prenatal androgen and oestrogen exposure. Most studies employing this method examine digit ratio for the right hand (R2D:4D) and/or left hand (L2D:4D), though the mean value (M2D:4D) (i.e., the average of R2D:4D and L2D:4D) and directional asymmetry (D[R–L]) (i.e., R2D:4D minus L2D:4D) are also commonly used. As no published studies have examined M2D:4D or D[R-L] in relation to testosterone measured from amniotic fluid, we conducted a secondary analysis of data published by Ventura et al. The sample comprises 106 mothers from Portugal who underwent amniocentesis during the second trimester and their neonates. Newborn M2D:4D was negatively correlated with amniotic testosterone in females (P<0.05) but not in males; no significant association was observed between amniotic testosterone and D[R–L] in either sex. In addition, we examined testosterone measured from maternal circulation during the second trimester, and found that it was not a significant predictor of M2D:4D or D[R–L] in male or female infants. Further research should aim to measure the ratio of testosterone to oestradiol present in amniotic fluid and maternal plasma, to examine whether either is a predictor of digit ratio variables at different stages of postnatal development.
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