<P> The endometrium is the inner epithelial layer , along with its mucous membrane , of the mammalian uterus . It has a basal layer and a functional layer ; the functional layer thickens and then is sloughed during the menstrual cycle or estrous cycle . During pregnancy , the glands and blood vessels in the endometrium further increase in size and number . Vascular spaces fuse and become interconnected , forming the placenta , which supplies oxygen and nutrition to the embryo and fetus . The presence of commensal bacteria in the uterus and endometrium has been identified . </P>
the inner lining of the uterus is shed during
during the menstrual cycle or estrous cycle