There are two layers that make up the uterus: the endometrium and the myometrium. The endometrium lines the uterus and it becomes thicker when a woman’s ovaries are producing estrogen. The myometrium is the muscular layer of the uterus and lies behind the endometrium.
The thickening of the endometrium is what makes it possible for the female body to support the growth of a baby. This thickening is caused by the hormone estrogen. After ovulation, the ovary produces another hormone, progesterone, which is intended to support the pregnancy. If pregnancy does not occur, the ovary will stop making progesterone causing the endometrial lining to slough off. The shedding of the endometrial lining is called menstruation, and happens monthly in the premenopausal woman.
Women taking estrogen for hot flashes or for birth control will develop thickening of their endometrium. Without a break from the estrogen, this thickness can become abnormal and develop into cancer. This is why it’s important to use progesterone or else have a limit on the time one’s body sees estrogen.