For example, a major reason that baby boys and girls come into the world with different brains and bodies is that baby boys produce high levels of the male hormone, testosterone, in utero.
Exposure to prenatal testosterone directs the cells in their growing bodies to organize themselves in the ways that we recognize as being male. They tell the reproductive organ cells to build penises and testes, and tell the brain cells to organize themselves in a way that predicts earlier gross motor development but later language stills. In the absence of prenatal testosterone, all babies look and act just like little girls, even when they’re not.