The umbilical vein is a blood vessel that carries oxygen-rich blood from the placenta to the developing fetus during pregnancy. It is an essential part of the fetal circulatory system, allowing the fetus to receive nutrients and oxygen necessary for growth and development. After birth, the umbilical vein typically closes off and becomes a fibrous structure known as the ligamentum teres. This transformation is part of the normal changes that occur in the body as it adapts to life outside the womb, marking the end of the reliance on the placenta for nourishment.