So I've scanned through the forums and couldn't find what i was looking for... I'm playing my first gen going onto my second gen, and I haven't really noticed any sims being introduced into family's ...
Without using any mods, unplayed Sims will often change jobs, and if your played Sim work as a doctor, some Sims will become pregnant as dealing with pregnancy is part of that profession. But the babies have unknown/non-exsisting dads. Maike Haas once had triplets in my game, because my Sim worked as a doctor.