To keep track of their cycle, your female sims can take a fertility test using the toilet (just like the pregnancy test) or use the app in their phone.
Pregnancies, as-is, are a little lackluster to me so here are a few of my favorite mods and some extra tips using MC Command Center and Wicked Whims (18+) that I think makes the whole pregnancy experience much, much better. My preferred style of play is legacy style which means there’s almost always someone in my sim family having a baby.