Answer? Two!

I have Free Time and with it you can have secondary aspirations. The top bit of 'Family' is 'fertility'. The chances of twins are -increased-.

That said, I have had this option chosen for two of my family Sims. One of them got pregnant twice, the other is in her first pregnancy.

All three pregnancies have resulted in - or will result in - twins.

Holy crap.

Soon it's going to be John and Kate + Eight in here! I thought increased fertility meant... y'know. Higher chances. But everytime? Daaaaaaaaang. I'm not complaining. It's just ... odd? Fun?