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I read this article which has the following quote:

The ratio between index and ring finger is believed to be linked to exposure to the male hormone testosterone in the womb.

And I wonder what determines the amount of exposure to testosterone in the womb. Is it testosterone produced by the fetus itself, in which case the baby's own genetics is the answer, or does this exposure the article speaks of more likely refer to testosterone produced by the mother and exposed to the baby?

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Fetal testis produces testosterone from cholesterol. There is a peak of production around 15 weeks of gestation (the "masculinization programming window"). So the genotype of the fetus can affect testosterone levels directly via effects on the biosynthesis of the hormone, or indirectly by defective regulation of the pathway's activity.

However, exposure to chemicals in the mother's environment is also thought to play a part in determining testosterone levels. The classic example of this is the phthalate esters used as plasticizers.

Scott et al. (2009) Steroidogenesis in the Fetal Testis and Its Susceptibility to Disruption by Exogenous Compounds. Endocrine Reviews 30: preprint freely available here.

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It has long been evident that human fetuses produce the hormones that dominate their development in utero. (Ref: Robinson et. al., J Clin Endocrinol Metab. 1977 Oct;45(4):755-61. Amniotic fluid androgens and estrogens in midgestation.)

There is a baseline increase in maternal testosterone during pregnancy regardless of fetal sex, and I can't find whether that is due to production by the mother's endocrine systems vs. transfer from the fetus. But in any case transfer from the fetus dominates: A male fetus produces so much testosterone that the serum levels detectable in the mother's blood ranges up to 4 times the amount found in typical non-pregnant women! (Ref: Meulenberg & Hofman, J Steroid Biochem Mol Biol. 1991 Jul;39(1):51-4. Maternal testosterone and fetal sex.)

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