The uterus (womb) is the pear shaped organ in the pelvis, at the top of the vagina, in which a baby grows. A layer of tissue called the endometrium lines the womb, and is designed to accept the implantation of a fertilized egg for the purposes of pregnancy. It is the endometrium which is shed every month in the form of a menstrual period, whenever a pregnancy does not occur. It is then replaced ready for another potential opportunity for pregnancy.