Making sure your sim doesn't have ugly random maternity wear: make an extra maternity-friendly outfit in each category and have your sim change into them
before they start showing/do the pregnancy spin as some call it. When choosing maternity outfits at random, the game will keep any maternity-ready things your sim is wearing when it happens so if everything's good for maternity, no changes needed.
On the other hand, Master Controller lets you drag pregnant sims to the stylist screen/CAS for outfit changes though you can only choose maternity-enabled clothes. I just wanted to explain the game's logic.
Maternity wear by pack: a few EPs (Generations, Showtime, Seasons) did include some maternity-enabled content.
Thank god people have already done the work for me. The image in the post I linked to includes some Store items and I can confirm that while they are in the maternity category, they don't give your sim a baby bump and are a curse when installed.
Maternity CC: there's actually a lot out there but my greatest hits:
OneEuroMutt's maternity defaults for all EPs, though please read the
issues with them too (nothing major just don't use his nude top/bottom replacements, some animations w/ the baby bump aren't great, and they can look weird on fat sims)
Buckley's maternity defaults for the Master Suite Stuff undies, also IMO her replacements for
University Life and
Late Night are a little more refined than OEM's and what I use personally.
Sweetdevil's edits (and age conversions) of EA content including maternity, not default replacement usually but she keeps them all in their own tag. She also adds maternity morphs for teen-elder if you have that need.
Tomislaw on The Sims Resource...I swear there was a less terrible site to download his stuff from but that may have been a false memory. Nice lil selection of maternity tops and outfits.
Last one's a "please bear with me" one: earlier in TS3's run the vast array of good maternity clothing was done by Poppy Sims (it was mostly retextures of the few pieces of base game maternity clothing but that goes a long way), who's site went down and
I quickly reuploaded some of her stuff without labeling what's maternity and what isn't (not my goal at the time I guess). And some time after that Poppy messaged me with what I believe to be more links but I'm having trouble getting the message or I'm wondering if I made it up entirely.