So I was an X-Files junkie and I never understood how there can be an alien fetus, when aliens seem to reproduce by black oil, (Infects, Gestates as reptilian alien, seeks heat to spawn as a grey alien.)
You have to keep in mind that there’s more than one type of alien on the show. One type changes their appearance (the alien bounty hunter, Jeremiah Smith, and the faceless rebels), then there are grays (which can be cross-bred with humans as they show in season 8), and then there’s the alien-as-a-virus business that’s associated with bees and black oil. It appears to me that the fetus is of a gray.
I was watching the final episode of Season 1 last night (episode ended with Deep Throat being killed and the X-Files closed down) and Scully had rescued a kidnapped Mulder by stealing an alien foetus and using it to trade for him.
Now I wasn’t paying much attention but… If I understood the rest of the episode it was about blending alien DNA with human DNA to produce hybrids. The hybrids had noxious green blood and could breath underwater.
So possibly, in that episode at least, the foetuses were hybrids deliberately and artificially created.