I know nothing about the Sims.
The bellies here look pretty good https://preggophilia.com/pregnant-belly-...gression-t, the bellies here are hit or miss https://preggophilia.com/sims-4-showcase...#pid364048, and the bellies here https://preggophilia.com/belly-presets-for-the-sims-4-t look god awful. 

Now I don't know anything about this Sims so for all I know these could all be using the same mod for pregnancy and the picture angles make them look better or worse.

All that said, does anyone know of any mods or ways to make good looking pregnant Sims (and while we are on the topic perhaps any other good adult mods)?

Thanks

At this point you either have to take the good sliders and presets that are already present or play the game with the default belly. The models in the game have such a low poly count that you're extremely limited with modifications without it looking like a blocky mess.

So when looks like this (from Hyashi123) it is done with out mods?

The second from the left looks like either a custom slider or preset. Either way if you want a round belly, stick without mods.