The walls of a uterus can thicken due to pregnancy or nonpregnancy-related issues, says Radiopaedia. The uterine lining of the uterus, called the endometrium, can thicken due to the various stages of pregnancy, tumors, cysts, an intrauterine blood clot or endometrial carcinoma.
Endometrial hyperplasia and endometritis are other reasons the walls of a uterus can thicken, explains Radiopaedia.org.The most common cause is the thickening of the uterus during days 14 to 38 of a woman's menstrual cycle, when it measures between 12 and 16 millimeters thick. During this time, progesterone, the hormone that prepares the lining for a fertilized egg, is released, explains the American Congress of Obstetricians and Gynecologists. If the egg is not fertilized, no pregnancy occurs, and the levels of estrogen and progesterone decrease. The monthly cycle starts and releases the endometrial lining.
An example of abnormal thickening of the uterine wall lining is endometrial hyperplasia, says Radiopaedia.org. Endometrial hyperplasia generally occurs in younger women who just begin to menstruate, and also in older women nearing menopause. Endometrial hyperplasia is when the body is overproducing estrogen, or when a woman is taking estrogen without progesterone. Regular pelvic exams beginning at age 18 or when a woman becomes sexually active help detect abnormal changes in the uterus.