The uterus, or womb, is the female sexual organ, in which the fertilized eggs develop into an embryo and fetus. All female mammals have an uterus.The following description refers to the human uterus.The uterus develops from two canals (paramesonephric ducts, Müllerian ducts) that fuse. If this fusion is incomplete the uterus can be seperated by a diaphragm or two uteri form (see anomalies).The uterus is relatively small before puberty, grows to 5-10 cm after puberty and extends cranially and laterally during pregnancy. It can then reach above the belly button. After delivery the uterus contracts and shrinks rapidly. After the menopause the uterus shrinks again.