If all goes well you get to pick two traits. Its not that hard to do, you just have to watch your pregnant sim a little more than usual. You can use lifetime rewards like Steel Bladder, which made my pregnancy go so much smoother because she didn't have to pee all the time. Another thing you have to complete as many of the pregnant sims wishes as possible, and read pregnancy books (they aren't required, but more times that not your sim wants to), maybe even go to the Hospital for advice. You have to really watch their needs, and make sure they are very comfortable and not on their feet for too long, as well.