Apoptosis can occur by change in hormone levels in the ovarian cycle. When there is no feilization of the ovum, the endometrial cells die because;

The stimulus for the corpus luteum to continue its function of progesterone production is LH. This stops after the 10th day of its formation in the absence of pregnancy. As the corpus luteum involutes, the levels of estrogen and progesterone falls and leads to a withdrawl bleed due to absence of progesterone that was suppoing the estrogen primed endometrium. Once pregnancy ensues , the hCG made from the syncytiotrophoblast is the stimulator for continuous corpus luteum function till around 12th week of gestation after which the placenta takes over the function of pregnancy maintenance.