A woman's uterus becomes swollen or enlarged as a result of pregnancy, with the size of the uterus growing from the size of a pear to the size of a watermelon over 40 weeks, according to WebMD. Additional causes for a swollen uterus include uterine fibroids and adenomyosis.
The uterus becomes enlarged or swollen when uterine fibroids are present, especially when the fibroids weigh several pounds. Uterine fibroids are noncancerous tumors within the muscular wall of the uterus, according to WebMD. Women may develop multiple fibroids that vary in size. With uterine fibroids, women often feel pressure in the lower abdomen, experience heavy periods, bleeding between periods, frequent urination, constipation and pain during intercourse, according to WebMD.
Another cause for a swollen or enlarged uterus is adenomyosis, a condition where the uterus thickens and tissue that typically lines the uterus moves into the muscular outer wall, according to WebMD. A growth called an adenomyoma then appears. The cause of adenomyoma development is unknown, though it is more commonly found in women who have undergone uterine surgery or a Cesarean delivery. In addition to an enlarged uterus, women with adenomyosis experience painful, longer periods, heavy bleeding and pain during sexual intercourse.