I know this is apt in a strict definition of the word, but do you use it to intentionally to convey a negative connotation? Yes, it is a parasitic relationship (the fetus needs the mother to survive), but keep in mind that the mother welcomed this relationship by, well, having sex. Sex is for procreation, not enjoyment, regardless of whether you take the evolutionary perspective or a religious/biblical one. It feels good to encourage population growth, otherwise, who would ever want to do such a thing (because admit it, sex is kinda icky). :D

So I don't feel bad for women. By having sex they are inviting this parasite to make a cozy home in the belly. Seems kinda crass to say 'hey, come on in so I can kill you'. (I know, that's a bit reaching...)