It doesn't really make complete sense, and here's my issue with it: If these young chestburster embyros can only be impanted into a female creature with a womb, um... yeah... that's just really, really weird. How would the aliens evolve a method of reproduction that depends on mammalian reproduction in order to function, especially when they are egg-layers? And, more importantly, they're ALIENS, and therefore how many freaking mammalian creatures could it really be encountering that would lead to such an adaption?! Weirder stlll when you consider that she specifically seems to target females that are already pregnant - even the waitress has a tiny bit of pudge and squeezes her stomach thoughtfully in one scene while thinking about her husband, which seems to indicate that she's pregnant too. That limits the options even further.