Various types of parasitic worms. Specifically, the really bad ones that breed massive colonies of themselves in your intestines. You remember all those pictures of the starving children in India/Africa who, despite the fact they were supposed to be starving, had massive guts? Well, it turns out they actually WERE starving, they just also had a fucking chestburster colony growing inside of them.
Personally, I think that if you're trying to get a potbellied loli, the pregnancy would come out both as the more erotic and the less harmful, but if you really wanted an alternative method, the worms are an option.