The thickening of the endometrium during perimenopause is known as endometrial hyperplasia, notes the American Congress of Obstetricians and Gynecologists. Though the condition usually occurs after menopause, it can occur before if ovulation does not occur regularly.
Women can develop endometrial hyperplasia when their bodies produce excess estrogen without producing progesterone, states the American Congress of Obstetricians and Gynecologists. The endometrium grows due to the estrogen, and ovulation may not occur due to the lack of progesterone. If the cells of the lining group together they become abnormal, leading to the thickening of the endometrium that can be a precursor to cancer in some women. Women who are over the age of 35, have never been pregnant, started menstruating at an early age and who are obese have an increased risk of developing endometrial hyperplasia.