Final answer:
When the corpus luteum disintegrates, there is a decrease in progesterone which triggers the start of the next menstrual cycle.
Step-by-step explanation:
When the corpus luteum disintegrates, there is a decrease in progesterone. This occurs if a fertilized egg is not implanted into the uterus, resulting in the degeneration of the corpus luteum. The decrease in progesterone levels allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the menstrual cycles again.