Harvard Medical School Guides
Harvard Medical School Guide: Help for Endometriosis
When a human egg is fertilized, it burrows into a specialized cushion of tissue that lines a woman’s uterus, called the endometrium. This lining, rich in blood vessels, not only serves as fertile soil for the growing embryo (later the fetus), but also helps to nourish the placenta, which provides food and oxygen. When a woman is not pregnant, she sheds part of the endometrium each month during menstruation.