Sexual differentiation of the human midtrimester brain.

It is firmly believed that sexual differentiation of the brain is linked with external genital differentiation in timing as an in utero event in the human. An extensive search for oestrogen, androgen and progestin receptors failed to show their presence despite adequate controls in cytosols from human fetal brain of gestational ages 14-20 weeks. It is possible that the receptors are present in levels so low that they are undetectable by present-day methods. Our results would indicate that hormonally influenced in utero brain sexual differentiation is most unlikely to occur as a mid-trimester event.