Look, I think it's bad enough that a pregnant sim who wishes to have a girl has to carry around up to five full-sized watermelons in their backpack no matter how athletic or out of shape they may ordinarily be and you want to increase their burden, so to speak? You are unkind and cruel, how could you even think of such a thing? :p
Hope I am using the correct terminology, but I would imagine what is required is more of a custom skin or maternity body mesh than an actual mod (in the scripting or tuning sense) that works on percentages. Tuning Mods or NRaas Retuner though could be used to make the baby bump such as it already is appear earlier in the pregnancy if that's any help.
