I've had something like this happen numerous times and finally I learned not to ever leave a pregnant sim during her first tri-mester and go play with another family.
Every time I did, the pregnant sim would either show up on another sim's lot or walk down the sidewalk of a community lot and when I'd go back to her she'd have her third trimester tummy, even though she was only in the second trimester. The first couple of times this happened I thought for sure I had a stuck pregnancy.
I'm only making a guess, but it could've been taking her to the community lot. I haven't installed the patch yet and have only had one pregnant sim in a while.