Possibility: baby is XY has 5-alpha reductase deficiency (5-ARD), doesn’t convert testosterone into dihydrotestosterone.
Development of sexual characteristicd is affected. Perhaps baby has no external testes, and external genitalia appear more female than male. Baby is declared a girl, raised as a girl.
Then at puberty, testes produce a huge surge of testosterone. The body masculinizes, muscle, bone,face, perhaps genitalia.
The child was raised as a girl in all good faith, likely thinks of herself as a girl, but her body grows into that of a man. Not her fault, but should s/he compete against women?
There is a population in the Dominican Republic that has a high number of people affected by 5-ARD, called guevedoces, “penis at twelve.” It that culture, the child is most likely to shift to a male identity.
All this nonsense detracts from the fact that they are not female, in the common sense of the term. It’s a tragedy and it’s too damn bad but if they went through puberty as boys then they should not be allowed to compete as females. Or simply repeal Title 9 (it’s virtually dead anywho…)