No even natural sunbathing is not ok. Yout internal temp rises faster than your skin. If you feel you have to tan then you wet a towel with cool water and lay it over your belly to keep your belly temp low and from baking baby.
Tanning naturally is fine as long as you wear sun screen, stay hydrated, and don't get over heated.
If your talking about in a tanning bed then no. It'd be safer to get spray tanned
Don't tan for long. Change in high body heat can effect baby
If your tanning at the beach and not in an actual tanning bed I would say its safe. But I would put sunscreen on so you dont burn.