The sudden enlargement of the belly when you are just pregnant is often clinically caused by the following reasons: 1. after you know you are pregnant, your activity decreases, your metabolism slows down, and you strengthen your nutrition in your daily life, so you eat relatively more, causing excess nutrition and fat accumulation. 2. because the skin of the abdominal wall is relatively loose, so fat tends to accumulate in the abdomen, resulting in the enlargement of the belly.