Endometrium

The endometrium is the mucous membrane that lines the inside of the uterus.

The endometrium changes throughout the menstrual cycle. About once a month, it thickens with extra blood vessels. A pregnancy can occur if a fertilized egg attaches to, or implants in, the endometrium. If a fertilized egg doesn't implant, part of the endometrium is shed, causing menstrual bleeding.