Hyperplasia, or thickening, of the endometrium, is most often caused by an excess of estrogen without progesterone, according to the American Congress of Obstetricians and Gynecologists, or ACOG. This condition usually occurs after menopause when ovulation ceases or during perimenopause when ovulation is irregular. In either scenario, since ovulation does not occur, progesterone is not made, and the endometrium may continue to grow instead of shed its lining.
Endometrial hyperplasia can also occur with the use of medications that simulate the way estrogen acts. It is sometimes seen with the long-term use of high doses of estrogen following menopause, especially in women who have not had a hysterectomy. Women with irregular periods, particularly when they are also infertile or have polycystic ovary syndrome, sometimes end up with endometrial hyperplasia. Obesity can also cause high levels of estrogen and related hyperplasia, according to the ACOG.
Women older than 35, white women and women who have never been pregnant are at heightened risk as well as women who experience menopause at an older age or who started menstruation early. The same is also true of smokers, women with a history of diabetes, thyroid or gallbladder disease, and those with a family history of cancers of the uterus, ovaries or colon, according to the ACOG.