Ooh, this kind of thread. I saw this one before but for some reason, I didn't participate early, so here's my two cents on the matter:
There are many factors to consider when determining an approximate projection of belly growth during pregnancy, which would include but not limited to the size of the mother and father prior to a pregnancy (both parties matter here as they'll be influencing the genetics of an unborn child; the bigger the father and the smaller the mother, the bigger her belly would be during pregnancy), the matter of the mother having her first or subsequent pregnancies (as stated by other before, each pregnancy is different, even to the same mother), if her diet is rich in sugars and carbohydrates (protein is also something to consider, but it's those two that creates fat to both the mother and unborn child, which also influences the birth weight of the child), if she's having multiples in a pregnancy (fairly obvious), if she's having prenatal conditions like polyhydramnios or diabetes mellitus (you wouldn't want that to happen to a pregnant woman), the current week of their pregnancy (again, fairly obvious), and the mother's general physique during pregnancy (from exposure to various sources, the more physically fit a woman is, the smaller their belly is during pregnancy).
So there you go. I hope that that should clear up some concerns on the matter.
There are many factors to consider when determining an approximate projection of belly growth during pregnancy, which would include but not limited to the size of the mother and father prior to a pregnancy (both parties matter here as they'll be influencing the genetics of an unborn child; the bigger the father and the smaller the mother, the bigger her belly would be during pregnancy), the matter of the mother having her first or subsequent pregnancies (as stated by other before, each pregnancy is different, even to the same mother), if her diet is rich in sugars and carbohydrates (protein is also something to consider, but it's those two that creates fat to both the mother and unborn child, which also influences the birth weight of the child), if she's having multiples in a pregnancy (fairly obvious), if she's having prenatal conditions like polyhydramnios or diabetes mellitus (you wouldn't want that to happen to a pregnant woman), the current week of their pregnancy (again, fairly obvious), and the mother's general physique during pregnancy (from exposure to various sources, the more physically fit a woman is, the smaller their belly is during pregnancy).
So there you go. I hope that that should clear up some concerns on the matter.