Added
It happened to my sim this week, with slight variations.
I left a sim pregnant with Father Winter's baby, and played her adult son's household. Afterwards, on the manage households screen her household clearly had a baby named Isabelle. I used the manage households to move out the ex-boyfriend who broke up with her after catching her with Father Winter, along with one of his sons. This left two spots in the household.
I played another household, and my sims ran into the mother on a community lot and my sim reacted as if he was seeing her go into labor. But she walked off the lot.
When I went back to her household, on the apartment selection screen the picture of her family had the bottle plus one to indicate there was a baby on the lot. But when I entered the lot, baby Isabelle did not exist, and mom was actively in labor. I played through the delivery, and she had twins. I have since left and re-entered that lot at least twice and Holly and Ivy have not disappeared. They are now toddlers. They both have the Father Winter's child trait.
Previous births while playing other households have been perfectly normal, even after Seasons was installed. Only Father Winter's daughter disappeared and was replaced.
Isabelle was born before I installed Get Famous and disappeared afterwards. I have all patches, expansions, and game packs and most of the stuff packs installed.
Forgot to include - when mom was first pregnant there were seven sims on the lot, so only one baby could be born. After the boyfriend and his child moved out, and Isabelle disappeared, there was room for three, but when I played through the labor mom had twins as usual (lot has ley line trait).
