What involves the breakdown of the endometrium?
The decrease in specific hormone levels.
In the final stage of the menstrual cycle, progesterone levels increase, progesterone is a hormone which thickens the endometrial lining in the uterus to nourish a fertilized egg. Though if pregnancy does not occur, estrogen and progesterone levels then decrease . The decrease in progesterone levels cause menstruation , or the breakdown of the endomentrium layer.