What causes the uterine lining to thicken?

The thickening of the uterine lining or endometrium is a natural occurrence related to menstruation and pregnancy. It can also be triggered by conditions, such as endometriosis, endometrial hyperplasia and uterine fibroids, that need medical attention, according to Everyday Health.

Throughout the menstrual cycle, the ovaries secrete varying levels of estrogen to prepare the uterus for pregnancy. Upon ovulation or release of the egg, another hormone is secreted, the progesterone, which makes the endometrium even thicker. Without fertilization, the levels of these two hormones will decline, triggering the sloughing off of the lining. The process is accompanied by bleeding, making it thinner and marking the start of a new menstrual cycle, states Everyday Health.