This lining is like a nest for the egg if pregnancy happens. After ovulation, the egg moves through a fallopian tube toward the uterus. Most of the time, the egg breaks apart before it gets there. Then the tissue and blood aren't needed. They flow out of the uterus, through the cervix and vagina out of the body.
Different women have different amounts of flow during their periods.
About only three or four tablespoonfuls of it are blood - although it may seem like more. But the egg doesn't always break apart. If the egg meets a sperm in the fallopian tube, they can join together. The joining of the egg and sperm is called fertilization. Pregnancy begins if a fertilized egg attaches itself to the lining of the uterus.