31 inches too big?!

I went to the OB they measured my belly. I am 29 weeks now and they measured 31. She asked her assistant "31 not 30?" i asked if this was bad or what it meant. She said he is just bigger than normal. I didnt go into for whatever reason. Now I am kinda worried. Does this mean he will have to come out earlier than the due date if this continues? Would I need to get a Cesarean if he gets too big by the time of delivery? Can I stop him getting too big by not eating fattening foods? I do have gestational diabetes so I am sure this contributed.