Similar thing happened to me. I have the life span on epic, so unless you manually age them up, they are nooboos for a long time. I'm into my 3rd generation of a family, and there are basically no teens or children for my sim teen and child to be friends with. So I decided I needed to populate the town, and swapped active household to a couple, made them try for baby, heard the chimes, went back to my main family. I saw the pregnant Sim several sim days later, thinking she'd have the big belly, but no, she had a toddler!