So far in out of all my saves, my sims had twins 3 times, two of which were just pure chance. No fertile reward. First time, my sim had two kids and her husband died, then she got pregnant from her new boyfriend. She had twin boys.
Second time, it was the first twin mother's grandson. He got a woman that lived in his house pregnant, and she had twins, a boy and a girl.
So, I was wondering what the chances of twins and triplets are without the fertile reward. In The Sims 3, my sims never had twins or triplets without doing anything special (kids music, kids tv, fertility treatment, ect.) In The Sims 2, my sims never had more than one without "forcetwins" or mods.