From what I remember from my last play, I used “Realistic Needs and Diseases” for weight gain instead of Weight Morphs. In addition to all of the other things it added (including bonuses/penalties based on your weight), RNAD also used the Weight Slider for weight gain instead of modifying the belly morphs directly. That way, it didn’t worry about conflicts with the pregnancy mod I was using at the time.
For best results, you still need to use CBBE or some Bodyslide-compatible body, and set up your skinny/heavy body sizes. You technically can use the default body, but it doesn’t get exceptionally large (you get a little stocky at most).