There is no one specific cause for a B-belly in pregnancy. B bellies are more common in people with more weight, but possible with any body size and shape. This is because so many unique factors, like height, weight, and muscle and bone structure all combine to impact how you carry a pregnancy bump. Ask your OB, additional health concerns may be further discussed.
Usually doctors are not really concern with B belly. However, if your weight is causing this then there might other health concerns to be discuss with your OB.