The pregnant girls use a slightly different texturing scheme than the regular girls. If you use a regular girl texture on a pregnant girl, her belly button will be super huge. Conversely, if you use a pregnant texture on a regular girl, her belly button will be tiny.
I am going to release (very soon, possibly today) a new set of base textures for pregnant girls on the site. Once it's up, I'll update the character creation thread with info on how to edit pregnant girls.