The endometrium is the lining that is inside the uterus. It prevents the middle layer of the uterus, called the myometrium, from developing adhesions, and provides an area for the placenta to attach to the uterus during pregnancy. It consists of a layer of epithelial cells joined to the myometrium by connective tissue.
The thickness of the endometrium changes along with the female's menstrual cycle. It thickens during the follicular cycle. It is during this period that the uterus prepares to accept a possible embryo. If an egg is fertilized and conception occurs, the placenta, which the embryo creates, will attach to the wall of the endometrium. This connection provides transportation of oxygen and nutrients from the mother to the embryo.
During most months, an egg will not be fertilized. In these months, the lining will break down and shed through the process of menstruation. The cycle then starts over again, as the body prepares itself once more for a possible pregnancy. When an embryo is present and implants in the uterus, the endometrium does not shed, rather it becomes part of the placenta, providing support as well as nutrients for the duration of the pregnancy.
Changes in the endometrium occur due to varying levels of hormones. The thickening of the inner layer of the uterus is a result of an increase in estrogen levels. After the fallopian tubes release an egg, progesterone levels rise as well. This stops the buildup of cells on the uterine wall, and prepares the uterus for possible implantation by an embryo. In months when no embryo is present, progesterone levels will once again drop, and the excess lining is shed through the menstrual cycle.
Sometimes, there are complications in the development of the endometrium. Low hormone levels sometimes prevent thickening of the inner uterine wall. The physical result of this is amenorrhea, or the absence of a menstrual cycle. It is most common in athletes or individuals with low levels of body fat. The average cycle for the endometrium to build up and shed is 28 days, but that can change as a reaction to stress, the seasons, and weight loss or gain.