The Fertile trait increases your chances of a multiple, but it's still a chance roll. And I don't know how much greater the chance is, but it doesn't seem to be much. I just gave that trait to a sim, and she had twins right after. And since, she's had 2 regular single pregnancies. Since there's always a chance of twins or triplets anyway, and werewolves have a higher chance of them-- the father was a werewolf-- I don't know that Fertile has had any effect.
I can tell you that the On Ley Line lot trait has a much greater effect. I'm pretty sure I've had multiples every time when the lot had that, except for when the household only had room for one more.
