The innermost lining layer of the uterus. It is the wall of the uterus where an embryo implants and a baby grows. It is an important part of the female reproductive system. It changes constantly throughout every menstual cycle from puberty to menopause. The endometrium contains numerous glands and is made up of two layers, or three layers, depending on which source you consult. The first layer is called the inner basal layer. It acts as a stabilizing anchor and remains unchanged during a woman’s menstrual cycle. The second and third layers are where a fertilized egg or a blastocyst will implant if conception takes place. They consist of a larger middle spongy layer and an outer compact layer. These two layers change in thickness depending on where a woman is in her menstrual cycle. They become thicker as ovulation approaches, in preparation for an embryo to implant if conception has occurred. The blood supply of the thickened lining provides nutrients for the embryo. If implantation does not occur, the thick lining breaks down and is sloughed off, and the blood it supplies is released. This creates a substantial part of a woman’s period flow. When a woman is using hormonal methods of birth control, her endometrium tends to be flatter and not nearly as thick. This is why her period flow might be lighter or there is barely any flow at all.