During the luteal phase of the menstrual cycle, estrogen and progesterone increase, and work together to create changes in the lining of the uterus that prepare it to accept an embryo, should conception occur. When conception (or pregnancy) does not occur, estrogen and progesterone levels decline, and the lining of the uterus, called the endometrial lining, begins to shed, which leads to menstruation.
The first of the two phases of the menstrual cycle is called the follicular phase

