The reasons for sudden weight gain and belly enlargement can be seen in the following: First, obesity, especially abdominal obesity. A sudden increase in the size of the stomach and weight gain is likely to be the result of obesity. Second, ascites. When hepatitis, cirrhosis, various causes of heart failure, or hypoproteinemia may occur ascites, the increase in ascites will lead to a sudden increase in weight, the stomach becomes larger within a short period of time. Third, abdominal tumors. When the patient occurs abdominal tumors, especially malignant tumors, may also lead to a sudden increase in the size of the stomach within a short period of time. Fourth, other. For example, abdominal flatulence can also cause the stomach to become larger. Therefore, if there is a sudden increase in the size of the stomach and weight within a short period of time, you should promptly go to the hospital for examination to clarify the cause.