The uterus or womb is usually about three inches long and hides behind the bones of the pelvis in the lower part of the abdomen. Under certain conditions, the uterus may become enlarged and can be felt arising from the lower part of the abdomen or pelvis. The uterus can actually grow from the lower part of the abdomen all the way to the edge of the rib cage.
Congenital Abnormalities
Congenital abnormalities or birth defects, according to the Merck manual, can cause enlargement of the uterus, particularly at puberty. Certain birth defects result in complete occlusion of the outlet from the uterine cavity, leading to retention of the monthly menstrual flow which balloons the uterus, a condition called hematocolpos, when the fluid swells only the vagina, and hematometra, when the fluid swells the uterus. The most common cause is an imperforate hymen where there is a failure of the hymen to develop an opening for the outflow of the menses. Other birth defects involving the cervix, uterus and vagina can also cause this blockade.
Pregnancy
The Merck manual cites this as the most common cause of symmetrical enlargement of the uterus in women of child bearing age. The baby growing in the womb causes it to enlarge till it practically takes up the whole abdomen. Sometimes, the pregnancy may go unsuspected for a long time. At the end of the pregnancy, the contents, baby, amniotic fluid and placenta, are expelled and the womb gradually shrinks back towards its normal size.
Fibroids
Fibroids or leiomyomas are benign tumors arising from the muscular wall of the uterus. According to the National Institutes of Health (NIH), they are the most common cause of uterine enlargement due to non-cancerous tumors in women of child-bearing age. They can grow as large as full term pregnancies. The tumors are classified according to their location and direction of growth in the muscular layer of the uterus. They are capable of causing uterine enlargement irrespective of their position. They can also completely distort the uterine cavity, interfering with the normal menstrual rhythm and further confusing them with pregnancy.
Cancers
Endometrial carcinoma, arising from the inner lining of the womb, can lead to the enlargement of the womb. According to the NIH, most endometrial cancers occur in women between 60 and 70 years of age, well beyond menopause. The first sign of endometrial cancer is bleeding from the womb after menopause.
Cancer of the cervix may also cause an enlargement of the womb. This is more common in younger women. The cancer mass may cause the womb to enlarge or block the outlet of the womb, causing it to swell with retained menstrual fluid.
Adenomyosis and Endometriosis
Endometriosis occurs when endometrial tissue, which usually stays in the innermost lining of the uterus, is transplanted elsewhere in the pelvis. When the endometrial tissue is found in the walls of the uterus, it is called adenomyosis. This is a benign condition which can occur as a complication of previous uterine surgery. Adenomyosis can lead to thickening of the uterine walls with an enlargement of the uterus.
Other Causes
Gestational trophoblastic disease or molar pregnancy, with its malignant counterpart called choriocarcinoma, can lead to the enlargement of the uterus. A pregnancy growing faster than usual is one of the first indications of this condition.
Cervical fibrosis and Asherman's syndrome two disease conditions capable of blocking the cervix. This leads to the accumulation of menstrual fluid within the vagina and the uterus. The accumulation of menstrual fluid leads to the swelling of the uterus.
Infections within the cavity of the uterus can lead to fluid or pus collection in the uterine cavity. This can lead to the enlargement of the uterus.
