Thickening of the endometrium, or the lining of the uterus, is caused by many different factors, some pregnancy related and some nonpregnancy related, notes Radiopaedia.org. Thickening of the uterine lining is caused by various stages of pregnancy, an intrauterine blood clot, cysts, tumors and endometrial carcinoma.
Other causes of endometrial thickening include endometrial hyperplasia and endometritis, Radiopaedia.org explains. Most commonly, the uterus thickens during days 14 to 28 of the menstrual cycle, and it measures between 12 and 16 millimeters in thickness.
One example of abnormal thickening of the uterine lining is endometrial hyperplasia, according to Radiopaedia.org. Endometrial hyperplasia happens in younger women who are just beginning to menstruate and in older women approaching menopause. Endometrial hyperplasia is caused by an overproduction of estrogen, or by taking estrogen without progesterone. To detect any abnormal changes in the uterus, women need to have regular pelvic exams starting at age 18 or once sexually active.