All foetuses have non-specific genitals for the first 8 weeks or so after conception.
Embryonic testes develop inside the body and start to produce androgens.
In AIS, these androgens cannot complete the male genital development due to a rare inability to use the androgens that the testes produce so the development of the external genitals continues along female lines. However, another hormone produced by the foetal testes suppresses the development of female internal organs. Internally, however, there are testes instead of a uterus and ovaries. A female body type with female external genitalia is the basic underlying human form. In about two thirds of all cases, AIS is inherited from the mother.