// I dunno, I was asking you as you seem to be AB’s resident trans expert. Surprising your not too clear on this basic point. I would have thought there was some legality involved as a sort of benchmark for future decisions on their gender.//
The words "assigned [male/female] at birth", though, are firstly quite standard in the topic these days. But I think it's fair to say that "assigned" applies more formality or thought than is really there. In most, I would say the vast majority of cases, out the baby pops (or up the 18-week scan comes) and everyone can see one way or another if it's a boy or a girl, and in such cases why would you need to think about it? And then that's what goes on the birth certificate.
On the other hand, in some proportion of births -- and, bearing in mind TTT's insistence that sex is entirely about chromosomes -- it turns out that either the genitalia are ambiguous, or mismatched to the chromosomes, or there will be some other set of sex chromosomes (XXX, XXY etc), and the visual "assignment" of sex is either wrong or misleading in another way. As far as I know, there's no legal rigour surrounding all of this, because for example intersex people often end up lumped into one sex or the other; and certainly there's no requirement, nor really should there be, for a chromosomal test for all live births.
And, likewise, since all of this is surrounding biological aspects of sex, rather than social aspects of gender, then although I read into it from time to time I would never wish to claim to be any kind of expert on it. As I hope the links in my earlier post show, though, there is more richness and complexity to the biology of sex than is often given credit for.