The endometrium begins to degenerate as the progesterone levels drop, initiating the next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again. What happens to the endometrium as the progesterone levels decrease?
1) It thickens to prepare for implantation
2) It sheds during menstruation
3) It remains unchanged
4) It becomes less vascular
