The Sims 4 the fertile trait don't work

Hello,

For multiple times now and multiple families I have been trying to get twins/triplets and when I first got the game I was getting twins every now and then but now I don't get any multiple births only singles, even with the fertile trait on one or both parents I still only get one child.

I was wondering if this was a bug or is it just hard to get multiple births?

Thanks.