The uterus is an inverted pear-shaped elastic bag-like structure. It is also called a womb. It prepares itself every month to receive the growing embryos. It nourishes the growing embryos by the thickened lining, richly supplied with blood called the endometrium. During birth, the muscle in the uterus shows rhythmic contraction. Every month the uterus sheds its endometrial lining which is known as menstruation which can last up to 3 to 5 days.