When the foetus is male (XY):
- The male hormone (testosterone) triggers the changes that change a male foetus into a male body, brain, sexual organs and sexual orientation.
- At around 6 weeks, when sexual identity is determined, the male foetus develops special cells which produce male hormones.
- There is a big surge of male hormones at adolescence (4 times greater than in infancy or boyhood).
- A female foetus exposed to male hormones in the womb cane have a 'male' brain in a female body.
- An rare abnormality of the adrenal gland results in secretion of a substance akin to male hormone, which often results in the formation of under-developed male genitalia plus a normal set of internal female reproductive organs. A high enough concentration can result in the baby being born looking male; so they are brought up as a boy, and the problem is only diagnosed when they fail to develop at puberty and genetic tests reveal they are XX.
- In Turner's syndrome, the child has only a single X chromosome, which results in exageratedly female behaviour, due to the fact that the ovaries of an XX female produce tiny amounts of male hormone.
