What hormonal change is responsible for the onset of menstruation?

The onset of menstruation is primarily triggered by a drop in estrogen and progesterone levels. During the menstrual cycle, after ovulation, the corpus luteum produces estrogen and progesterone, which help to prepare the uterine lining for a potential pregnancy. If fertilization does not occur, the corpus luteum degenerates, leading to a decrease in these hormones.

This decline in estrogen and progesterone levels results in the breakdown of the uterine lining, which is a key event in the initiation of menstruation. Without sufficient levels of these hormones, the support for the uterine lining is lost, ultimately resulting in its shedding, which constitutes menstruation.

The correct understanding of this hormonal interplay illustrates why the drop in estrogen and progesterone is crucial for the menstrual cycle's functioning and the initiation of menstruation.