A normal-sized uterus is about the same size and shape as a pear and weighs 6 ounces, according to the National Uterine Fibroids Foundation. It is 3 to 4 inches long and over 2 inches wide. During pregnancy, the uterus can expand to the size of a watermelon, says WebMD.
The uterus may become enlarged for reasons other than pregnancy. Uterine fibroids and adenomyosis are the two most common causes of an enlarged uterus, according to WebMD. Fibroids are noncancerous tumors that attach to the muscular wall of the uterus, affecting nearly eight in 10 women. Adenomyosis is a thickening of the uterus caused by movement of the uterus lining into the outer muscular wall.