At Nutella, As It turns out my son was not an alien at all , but possibly some weird glitch.
Sadly, I am beginning to think that Twinmum is correct, at least for me. Every time I have two sims "try for baby" they end up with twins. One being carried by the mother and one by the father. It always acts as if it is an alien birth, yet nothing about the child turns out be alien...