The main cause of most endometrial cancer is too much of the hormone estrogen compared to the body's progesterone level.²

Estrogen makes the lining of the uterus (endometrium) grow thicker. Progesterone "opposes" estrogen-your progesterone level goes up then drops at the end of each menstrual cycle, making the thick endometrium layer shed away. This is what you know as menstrual bleeding.

When there is too much estrogen in the body, progesterone can't do its job. The endometrium gets thicker and thicker. Over time, the endometrium cells can become cancerous.