As the title says. Some of my Sims moved in together (two are sisters, they were born in-game) and the welcome wagon showed up. One member was Brytani Cho, who is an Elder. She's apparently pregnant! I have no idea how long she's been an elder, I've been playing this save a while. I googled this, and it appears to have happened to some people before. The only mod I have that has anything to do with pregnancy is MCCC, the latest version following the last patch.
Using MCCC I discovered that she's in her second trimester with a girl.
Sims 3 Families: Sanchezes, Fairy Friends, Jenningses
Sims 4 Families: Fosters/Matsumotos/van Beerses, Harper-Joneses, Mastersons