The uterus, or womb, is the place in the body where a baby grows. Located in the pelvis, the female reproductive organ can become enlarged for a variety of reasons. The most likely cause depends on a woman's age. In women of childbearing age, pregnancy -- either normal or abnormal -- is the most common reason for an enlarged uterus. Benign uterine tumors, known as fibroids, can also be a cause. Occasionally, in young women who are just starting their periods, a blockage of menstrual blood flow leads to an enlarged uterus. In older women, cancer involving the uterus is a potential cause for enlargement.
Within the first few weeks of a normal pregnancy, the uterus begins to grow. It starts at a weight of approximately 70 g and grows to about 1,100 g at term. A miscarriage, when a fetus or embryo is no longer living, or an ectopic pregnancy, when a pregnancy is implanted outside of the uterus, can also result in an enlarged uterus. Yet another type of abnormal pregnancy, called a molar pregnancy, can lead to excessive uterine growth.
Uterine fibroids, or leiomyomas, are the most common tumor found in the pelvis. According to the American College of Obstetricians and Gynecologists, uterine fibroids occur in up to 70 percent of white women and 80 percent of black women by age 50. They are rare in adolescence and usually shrink after menopause. They can be so small that they are only visible with a microscope, or in rare cases they can exceed 25 lbs. Although most of the time they do not cause symptoms, they can cause abnormal bleeding, pelvic pressure, painful periods and urinary frequency.
Adenomyosis and Hematometra
Adenomyosis occurs when the endometrium -- the lining of the uterus -- grows into the walls of the uterus, causing it to grow up to 2 to 3 times normal size. It affects approximately 20 percent of women. Classic symptoms of adenomyosis are heavy, painful periods, but -- like fibroids -- it is most often asymptomatic. Also like fibroids, it affects women of childbearing age. Hematometra is another disorder that can cause an enlarged uterus. It occurs when the uterus expands with blocked menstrual blood flow. This can happen in young women who are just starting to menstruate or in older women who have scarring within the pelvis, usually from radiation or surgery.
Cancer involving the uterus can cause enlargement. The most common cancer of the uterus is endometrial cancer. It will be diagnosed in approximately 54,870 women in the United States annually, according to the American College of Obstetricians and Gynecologists. Rarely, cancer occurs within the muscular wall of the uterus. This is known as a sarcoma. Cancer can also occur within a fibroid -- this is called a leiomyosarcoma. Cancers are most likely to occur in postmenopausal women.