Why this sort of thing isn’t standard in maternity wear I don’t know. But it’s pretty easy to DIY, even if you don’t sew.
Because when I get to a certain point in pregnancy my shirts ride up. Or generally get too short. Perhaps I’m disproportionate, but I’m also short with a short torso, so you wouldn’t think that maternity shirts would be too short.
Anyhow. Here’s what you do:
Take any shirt with a bottom hem. Cut a hole. If the shirt has side seams I cut one small hole near each side seam.
Measure yourself a piece of elastic. I like this thinner type, but really whatever fits in your seam. I pull a piece from hip to hip making sure it has enough tension. Basically not loose.
Insert elastic into hole and thread through the seam. A good way to do this is to use a safety pin. I’ve also used crochet hooks and seam rippers. Safety pin is probably easiest though.
Sew the holes and elastic up on each side.
You should now have a elasticated hem in the front of your shirt.
Look, it stays put! (on my not even 30 week belly that has outgrown some shirts already)
I have made several undershirts like this as well as altered some other maternity shirts. No riding up here!