The highly vascularized, modified mucosal lining of the endometrium that is established through the process of decidualization. If an embryo does not implant in the decidualized uterus, the lining of the uterus is shed during the next period of menstruation. However, if implantation occurs, the decidua is maintained, and becomes the main source of nutrients for the developing fetus throughout the early weeks of pregnancy, until placenta becomes capable of assuming this responsibility.