I have a pet theory that the Little Ones were in utero when Z-day hit. The blood smears and tattoo gun left at Cain Memorial hospital (it was described as a general hospital, so likely to have some form of maternity facilities) are what started me on that. Ink either foun biters who were pregnant, or some un-turned pregnant women and...experimented. And how do you keep track of lab rats? Tattoos.

I might be getting too here, but the speculation's too much fun to stop!