I think I know what the problem might be, in 3D models the waist area is usually influenced by 2-3 bones which causes a pregnant belly to fold unnaturally. Normally you could weigh most of the belly to one bone to address the problem, but that doesn't work if the game has a dynamic belly size.
I'm not sure if this is feasible but if you're willing to sacrifice the dynamic belly size, you can probably make 3-4 versions of the model with a preset belly size and appropriate weighs, then swap between those models when egging / birth. Can probably hide it with a fade to black. Though I'm no expert and there might be some magic solution I don't know about lol.