Noun | 1. | endometrium - (pregnancy) the mucous membrane that lines the uterus; thickens under hormonal control and (if pregnancy does not occur) is shed in menstruation; if pregnancy occurs it is shed along with the placenta at parturition mucosa, mucous membrane - mucus-secreting membrane lining all body cavities or passages that communicate with the exterior uterus, womb - a hollow muscular organ in the pelvic cavity of females; contains the developing fetus decidua - the epithelial tissue of the endometrium |