During childbearing years, one of the ovaries releases an egg each month. This is called ovulation. The ovaries produce hormones each month that prepare the uterus to nourish this egg, if it is fertilized. Estrogen is produced in the ovaries during the entire menstrual cycle. It prompts the lining of the uterus, the endometrium, to grow a thick layer of tissue each month.
Progesterone is produced in the ovaries during the second half of the menstrual cycle.
It further thickens the lining of the uterus. If the egg is not fertilized, it moves from the fallopian tube to the uterus and is absorbed. The levels of both of these hormones then drop. This signals the uterus to shed its lining. This shedding is your monthly period.