Before birth is every fetus a boy?

Answer 1

'Before birth' a fetus could either be a boy or a girl (I'm just going to assume you mean before the woman goes into labour) The expression of the fetus' sex occurs around six weeks after conception. Before gonad differentiation the fetus has no sex (no testis or ovaries at this stage, so 'no sex'). After gonad differentiation it will either be a boy (testis) or a girl (ovaries).

To be more specific, every fetus starts out in a feminine direction unless several genes on the Y chromosome exist and trigger sex-differentiation in the other direction. So by "default," everyone would end up a girl if it were not for certain proteins on the Y chromosome (or technically even an X if it had these genes instead of the corresponding ones which usually exist there).

There are some males which are conceived missing the receptors for male hormones. So despite the sex-differentiation cascade, the fetus would develop outwardly as female, since although the Y-chromosome causes the adrenals to produce DHT to trigger the male differentiation, the body cannot use the DHT, and the male structures never completely form. Because of that, there are no inhibitors released to stop the development of female genitalia. But because the genes for female gonads does not exist, such girls end up with shallow vaginae, undescended testes (which are of no use to the body since they lack male hormone receptors to use testosterone), and delayed puberty. They do have female puberty because unused male hormones converts to estrogen in the body. That is something that steroid users learn the hard way, when they end up with feminine breasts in addition to muscles. They grow taller than other girls because the estrogen comes late. Usually, such girls are well-adjusted into the female role, and they may be given hormones and surgeries to make their features more typical for women.