US citizenship for babies born overseas to American moms: foreign zygotes need not apply

[Grace62]

The law says that if a US citizen is overseas when her child is born, that child has US citizenship. Where does it say that the egg and sperm have to have come from US citizens?
I'm very surprised by this, I would have assumed that the child would have citizenship, maybe unless the child was immediately adopted by non-citizens, such as in a surrogate case.