I've been abducted 3 times with my Adult female sim, and she hasn't gotten pregnant yet!
for some reason the aliens like the men better. they get pregnant alot easier. type in the boolprop cheat as it was stated above and search for spawn... tombstone of L&D. a tombstone(duh ) should appear somewhere on the lot near your sim and click on it and search for make me ALIEN pregnant. it sould work. if its not there play around with boolprop a little more in the spawn area. don't use it too much though or lots of errors will start to happen and it gets really annoying.