Even though I use a mod to let pregnant sims wear any clothes, as I'm sure almost everyone does, pregnancy defaults still serve a valuable purpose for me. They prevent stuff like this:
Well, not the background pink soup, or my dumb lot names, but the hideous Maxis maternity outfit on the family load screen.
Yeah, her palla still clashes with it, but at least it looks like the kind of thing a woman might actually do when she's eight months pregnant and she just wants to waddle out through the garden to get some fresh air.
The dresses are all from Tipsypixel's Ladies of the Dark Ages set, and I primarily chose from the ones with copper brooches (I think one or two have silver) and more subdued dye colors, on the grounds that it's easier to justify a higher-class person dressing down than vice versa.
Everything seemed to be working fine, but I didn't have enough pregnant ladies on hand to test every color, so let me know if something's messed up.
Compressorized, all in one file, download at SFS or Box.