The endometrium is the inner lining of the womb, richly supplied with glands and blood vessels.
The endometrium changes during the menstrual cycle. Every month the womb prepares for the implantation of a fertilized egg. The spongy tissue that lines the inside of the uterus is filled with blood, able to supply nourishment.
When no fertilization takes place, this preparation was in vain. The lining, which is no longer needed, is shed. It leaves the body through the vagina: the girl has her period.
When an egg is fertilized it settles in the uterus. The lining is not shed and the girl won’t have her period: she is pregnant.