Consider: Both times Cordy (R.I.P.) was taken over by a demonic personality, it was through a pregnancy. Oh, and there was also that episode in which demons impregnated her and she grew an eye in the back of her head. (Ew.)
In the Joss-scripted Alien: Resurrection, Ellen Ripley becomes “the monster’s mother” and gains alien characteristics as a side-effect of the cloning process that creates a viviparous alien queen—and was it just me, or was Fred was totally having a Ripley moment when she was wielding that flamethrower?
Oh, I should make a distinction between these cases of possession and the Angel/Angelus and Good Willow/Bad Willow situation. In the latter, it’s a matter of their repressed dark sides coming to the fore, as opposed to their being taken over by external personalities.
So, am I onto something, or am I just short on sleep? (Actually, I’m definitely short on sleep, but the first half of the question remains.)