The uterus is a muscular organ located between the bladder and rectum in the female pelvis. The uterus contains the endometrium, which sheds during menstruation and nourishes the developing fetus if a pregnancy occurs. The uterus enlarges naturally during a pregnancy, but an enlarged uterus can also occur due to certain medical conditions affecting its muscle or tissue.
Pregnancy
Pregnancy occurs when a sperm fertilizes an egg, and in a normal pregnancy, the fertilized egg implants itself in the uterine wall. A pregnant woman's body goes through a number of changes during the various stages of pregnancy. Some women notice abdominal bloating early in the pregnancy. According to Cedars-Sinai Medical Center, in the first four weeks of pregnancy, the uterus appears enlarged and irregularly softened. At 20 weeks gestation, the uterus enlarges and the upper point of the uterus sits at the level of the woman's belly button. The uterus continues enlarging to accommodate the growing fetus, and at 36 weeks gestation, the upper point of the uterus sits near the bottom of the breast bone.
Adenomyosis
According to Medline Plus, adenomyosis occurs when the endometrial tissue, which normally lines the uterus, thickens and moves into the outer muscular walls of the uterus. Adenomyosis occurs in women older than 30 who have had children, and who have had a previous Cesarean section or other uterine surgery. Adenomyosis may cause an adenomyoma, which appears as a mass or growth within the uterus. The condition causes an enlarged uterus and the doctor usually notices the enlargement during a pelvic exam.
Uterine Fibroids
Uterine fibroids, also called leiomyomas or myomas, develop from the muscle tissue of the uterus and appear as non-cancerous growths, reports the American Congress of Obstetricians and Gynecologists. Uterine fibroids remain small and do not cause symptoms or problems, but they can cause problems depending on their size, number and location. Fibroids can occur inside the uterus, on its outer surface, within its wall, or attached to it by a stem-like structure. Uterine fibroids can happen at any age, but occur most often in women aged 30 to 40 years. Fibroids cause an enlarged uterus and the doctor usually notices the enlargement during a routine pelvic exam or during tests for other problems.
