I've certainly referred to those creatures in-utero as "parasites"

[reynardo]

particularly in the way they're completely able to make sure *they* get the best of everything, even when it leaves the host-mother drained and exhausted and going through all sorts of complications. (That doesn't mean I'm against babies - I'm just in awe at how well they've set themselves up)

So Pharyngula's article on why women menstruate was utterly fascinating. If women have to put up with that crap every month, at least it's useful to know it's for our own good and protection!