Definition: The endometrium is the lining of the inside of the uterus. During the follicular phase of the menstrual cycle, the endometrium thickens in preparation to accept an embryo should conception occur. If pregnancy occurs, the endometrium undergoes a process called decidualization. The placenta, which is fetal derived, invades the endometrium to allow for the transport of nutrients and oxygen from the mother to the fetus. If conception does not occur, the endometrial lining begins to breakdown and sheds through menstruation.