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Fetuses don't actually start as 'female'. Zygotes start with a chromosonal gender. The fetus then develops along what appears to be female lines for a few weeks, with both 'male' and 'female' fetuses showing 'ovaries'. When the fetus is exposed to testosterone (I think around week 8), the 'male' changes, with the 'ovaries' descending and developing into testes. The external genitalia also changes, with male developing a penis. In some rare cases, a 'male' zygote will develop into a fetus which is immune to testosterone. In those cases, the fetus never goes through that development from female looking physiology to male. The testes remain in the place where a female's ovaries are, and the penis never develops. They appear to all intents and purposes female. But, the 'ovaries' really are testes. And they are still chromosomally male.
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