Yes but my mom is pregnant sounds more appropriate😅

My mom carried a baby in her womb. That is what I would say 🙃

There was a baby in my mom’s belly. Or my mom was pregnant

Both are accurate but too much is left to the imagination and it gives the impression that perhaps your mother enjoy ingesting children. Pregnant is a word specifically for carrying a child in the womb. If she’s currently pregnant: My mom is pregnant. If she has already delivered the baby: My mom was pregnant.

“Belly” or “womb” would be correct, but it sounds a bit awkward and maybe even inappropriate in some situations. I would just say “My mom was pregnant” or “My mom was pregnant with the baby”.