What triggers the end of the luteal phase in the absence of pregnancy?

The end of the luteal phase is primarily marked by a decrease in the levels of estrogen and progesterone. During this phase, after ovulation, the corpus luteum forms and produces these hormones, which are essential for maintaining the uterine lining for a potential pregnancy. If fertilization does not occur, the corpus luteum begins to degenerate around day 14 of the luteal phase, leading to a decline in progesterone and estrogen levels.

This hormonal drop is crucial because the decreased concentration of these hormones removes the hormonal signals that were maintaining the uterine lining, thereby leading to menstruation. In essence, it's this decline in progesterone and estrogen that signifies the end of the luteal phase, allowing for the cycle to start anew.