| If an egg is not fertilized and implanted, decreases in progesterone and estrogen lead to a decrease in thickness of the endometrium (lining), with glandular collapse, and spiraling of the vessels that supply blood to the uterine lining. As the arterioles spiral and become coiled, blood flow is reduced and the lining becomes ischemic, lacking oxygen. The endometrium thins out, and then, during the menstrual phase, there is bleeding from the arterial and venous systems of the uterine lining. | | | | |