What a pretty mama! I appreciate the creavity with the concept and the fun colors here.. Overall, I feel like there is not enough focus on the belly, especially with the crops you're showing. Here are my individual thoughts:
1 - I'd back off the retouching and the contrast and saturation. The colors seem plenty bright enough without too much pushing. I'd also lighten up on the vignette and I think there could be a better than square crop. You've got some awkward cropping here, cutting off both the bottom of her belly and cutting her off right at the wrist.
2. Here, again, you have it cut off the bottom of her belly. Her head is dead center in the frame so you have all that building in the background that you don't need. Also, it would be nicer if we could see her hands versus hiding them behind her body. Also, similar notes about processing.
3. I would definitely avoid the selective color.. This one to me actually has the most focus on the belly but I would prefer just a color process.
4. Similar composition issues here.. At least we have the whole belly but with pregnant women you want to have them angle their hips the other direction, because it's more flattering. Also a bit over processed.
Keep on practicing!