When kids can't talk, they can't self-express gender, so you pretty much have to go on biological sex assigned at birth. I suppose you could do a retrospective study, but it would take 20+ years to get that data and you'd still have sample size issues and dropout issues where you couldn't get accurate self-selected gender information for people because they didn't update contact information.

Unfortunately, there are some parts of developmental science that are just plain difficult to study with modern understanding of gender identity.