The uterus is a pear-shaped muscular organ that measures roughly about 7.5cm in the non-pregnant state. It serves as the vessel for a baby's growth and development during a normal pregnancy. The uterus is not usually palpable, or felt, as it sits behind the bones of the pelvis. Certain conditions lead to the enlargement of the uterus beyond its hiding place, making it possible sometimes to feel it in the lower part of the abdomen as an abdominal mass.
Birth Defects
Birth defects usually cause uterine enlargement in the younger generation, especially around the onset of puberty. The most common cause of uterine enlargement from birth defects is a failure of development of an outlet for menstrual flow. One common defect is an imperforate hymen which is characterized by the obstruction of menstrual flow from the uterus. With successive cycles, a ballooning out the uterus occurs, a condition called hematometra. A surgical procedure is usually required to correct these defects.
Pregnancy
According to "Merck Manual," this is the most common cause of a lower abdominal (pelvic) mass in women of childbearing age. The whole process of pregnancy leads to a gradual enlargement of the uterus. As the contents of the uterus (the fetus, membranes, placenta and amniotic fluid) increase in size, the uterus swells and also increases its muscle bulk. Thus, at the end of pregnancy when all the contents have been expelled, the uterus remains bulky for a long time and gradually returns to its former size.
Fibroids
The National Institutes of Health names fibroids as the most common benign growths of the uterus, arising from the musculature of the uterus. They can grow as large as a full-term pregnancy, distorting the normal anatomy of the uterus and sometimes disrupting normal menstrual rhythm. They are typically classified according to their position in the walls of the uterus. All kinds of fibroids have the potential of causing an enlargement of the uterus
Endometriosis and Adenomyosis
Endometriosis is the transplanting of pockets of the lining of the uterus, the endometrium, onto sites outside the cavity of the uterus. When this affects the walls of the uterus, it is called adenomyosis. With successive menstrual cycles, these pockets swell with menstrual fluid, causing the uterus to enlarge. The National Institutes of Health says it is more common in women with previous uterine surgery.
Gestational Trophoblastic Disease (Hydatidiform Mole)
Gestational trophoblastic disease, also known as molar pregnancy, also leads to enlargement of the uterus. The condition is due to an abnormal development of fetal tissue and is characterized by sudden uterine enlargement. The enlargement is usually faster than that of a normal pregnancy. According to the National Institutes of Health, it is more common before the age of 16 years and after the age of 40 years.
Cancer
Uterine cancer is typically seen in older women after the onset of menopause. According to the National Institutes of Health, the most common type usually arises in the lining of the uterus, the endometrium. The usual presentation is bleeding from the vagina after established menopause. It also causes uterine enlargement.
Cervical cancer, cancer arising where the uterus opens into the vagina, also leads to uterine enlargement, either by its bulk or by blocking menstrual flow and swelling up the uterus with the back flow.
Other Causes
There are other possible causes of uterine enlargement such as Asherman's syndrome and cervical fibrosis. These can cause an occlusion in the outflow of menstrual fluid, leading to an enlargement of the uterus. Infections with pus or fluid collections can also take place in the uterus, leading to an enlargement.
