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No, that' s not what happens, and that argument is actually used by TIMs to claim femalehood.

I thought every fetus started developing as female until the testosterone kicked in. I always knew sex was determined at the moment of conception by the sex of the mans sperm cell but don't all fetuses start developing as female?

We start with similar structures, so similar that they are often mistaken for identical and that' s where the idea that we all start as females comes from. I remember a picture in one of my biology books from years ago in which there was a colour scheme of these structures in a male and a female embryo, and there are very little differences that grow in dimensions and shape with the passing of time and the development of the embryo.

Then, at one point, there is a shot of hormones that male embryos react to by starting to develop male sexual characteristics: this reaction happens because of the presence of the Y chromosome.

Long story short, no, we don' t start all as females, you can' t be female if you have a Y chromosome because that means you are destined, if everything works, to develop a male reproductive system. If your body doesn' t react to these hormones even if you have a Y chromosome, then it develops similarly to a female body when it comes to superficial structures and secondary sex characteristics, but it' s not completely identical even in that case and of course you don' t get to have a female reproductive system.