Thickening of the uterine wall during perimenopause is caused by excess estrogen and a lack of progesterone, states the American Congress of Obstetrics and Gynecologists. When the uterine lining is not shed or shed intermittently, progesterone is not created. Some conditions or medications can also increase estrogen production.
Using medications that mimic estrogen and being obese can contribute to this condition, which is referred to as endometrial hyperplasia, according to the American Congress of Obstetrics and Gynecologists. Women over age 35 and those who smoke cigarettes, begin menstruating at an early age or are an older age at menopause are at higher risk of developing for developing this condition.