I was playing for a bit last night, rather than editing, and Donna ended up pregnant.
That's the third set of twins out of six pregnancies so far this generation, and every set has belonged to a sim that is a twin in my story. (Sophia, Andrew and now Christopher). Now I know that Sophia and Lauren were never really twins in the game; they're ordinary ALT dormies, but I decided to make them twins for my story since they are identical. I also know that both Andrew and Christopher, although twins from Regalton 2.0, were created as single sims in Regalton 3.0 and so shouldn't carry any twin gene, but I don't think my game knows that. It tickles me that my game is making my sims from multiple births have twins themselves.
I've also decided that playing just two days in each house for a rotation is too short. I'm going to give playing four a go next time, and have been trying to come up with a solution for playing university to keep it all in line with the rest of the neighbourhood. At the moment I'm thinking of playing the university lots both at the start of the rotation and at the end. This should allow me graduate any sims who will graduate after the first year of the rotation and be able to put them into the hood at the proper time, whilst also allowing me to send any sims to uni after the first year who should go, and play through their first year at uni.