The endometrium is the innermost lining of the uterus. This mucosal tissue is described as the “wallpaper” of the womb. It goes through a sequence of regeneration as evidenced by the menstruation cycle. The endometrium becomes thicker just before ovulation to prepare for the possible implantation of the fertilized egg. The lining is then enriched with blood to support the zygote and the placenta. If pregnancy does not occur, the unnecessary tissues come out as menstrual flow.