The uterus is the female sexual organ in which the fertilized egg cells ripen from embryo to fetus until birth.
All female mammals have a uterus, but the following refers to humans.
During development the uterus forms out of two tubes that fuse together. If the fusion is incomplete the uterus can possess are smaller or bigger septum or in rare cases two uteruses can form.
Before puberty the uterus is relatively small and afterwards reaches 5-10 cm. During pregnancy it expands outwards and can reach over the naval. Post-partum the uterus retracts rapidly. After menopause the uterus shrinks further.