What hormonal change triggers the beginning of menstruation?

The onset of menstruation is primarily triggered by a decrease in progesterone levels following the luteal phase of the menstrual cycle. After ovulation, the corpus luteum forms and secretes progesterone, which helps to prepare the uterine lining for a potential implantation of a fertilized egg. If pregnancy does not occur, the corpus luteum degenerates, leading to a significant drop in progesterone. This decline in progesterone is crucial because it results in the shedding of the uterine lining, which is what constitutes menstruation. Without enough progesterone to maintain the thickened lining of the uterus, the body signals for the lining to be expelled, thus starting the menstrual cycle anew. Understanding this hormonal interplay is essential for grasping how the menstrual cycle operates as a whole.