I just had a sim spin into maternity wear - for everyday, she got a hoodie from Generations and the base-game jeans, for formal she got a peasant blouse with a sash and stretch pants, and she's got a Generations T-shirt and the base-game exercise pants for sleepwear. Athletic was the same-ol' long-sleeved tee from the base game, and swimwear was the V-neck one-piece.
I'm strangely excited that I'll be able to tell my formal from my nightgown without checking for shoes.
Should we all share pics to help pick out customizable stuff? Unless we've got someone playing Baby Boomer handy, I don't think any one person is likely to find all the options, but collectively I bet we have a lot of pregnancies in progress.