Honestly, not much -
There are many myths about the size, shape and weight of pregnant bellys - but they are more for fun then for fact. That is why they are called, "Myths".
I know my four pregnancies all were different in their own ways and I had 4 girls - the first two were 6lbs 10oz, my 3rd was 8lbs 3oz, and my 4th was 8lbs 2oz. Even though my first two smaller babies were about the same in size and weight and my last two little girls too, my belly size and shape were different for all four of them. I can also tell you that I carried each one of them so much differently that I would have pains from walking, sitting, standing, even sleeping in different places for each child.
One thing that was true for my last two daughters was how small my belly was up until my 7th month and then whamooo out came a big belly.
Here are a few sites to read more about pregnant bellys.
Baby Zone..... Your Growing Belly: What Your Bump Reveals about Baby
The Well-Rounded Mama - Some interesting facts about your pregnant belly!
Your growing belly - Check out this gorgeous, month-by-month photo gallery of maternity images by photographer Sarah Merians. See how your belly grows -- and see how your baby grows!