There is growing evidence that there are multiple gender "events" in utero, aside from the determining of genetalia. I used th word "events" because I cannot recall the language used in the few articles I read (at least one of which was on the NIH website). But, broadly, there are hormonal impacts in utero that may be linked to transgender identities. If I understood it correctly, the research seems to be saying that hormones active at varioius times in utero can assign one gender to genetalia and another to the brain or brain chemistry.

I'm sorry. I know I'm over simplifying the research I read. It's been some time ago and it was rather over my head scientifically.