The uterus is the female sexual organ in which fertilized egg cells become an embryo before birth. All female mammals have a uterus. On the following just the human uterus is described.From an evolutionary point of view the uterus develops from two channels, the Muellerian ducts, which fuse together. If this fusion does not occur completely, the uterus may have a septum or even duplicate.Before puberty the uterus is rather small, grows then to a size of 5-10cm and can expand during pregnancy to all sides. It can expand up to the belly button.After giving birth, the muscles of the uterus contract in a very short space of time and thus make the organ small again.After the menopause, the uterus gets smaller as well.