According to Family Education, the umbilical arteries transport wastes, such as carbon dioxide, from the fetus to the placenta. Fetuses have two arteries that travel through the umbilical cord and one vein. The vein transports fresh blood, containing oxygen and nutrients, from the placenta to the baby’s body.
Umbilical cords are about 23 inches long and coiled to allow the baby some freedom of movement. The cord usually develops the coil by week nine, according to Family Education, but sometimes it takes until week 20 to coil completely. Sometimes, the three vessels of the cord separate prior to their junction with the placenta.
The placenta is a unique blood vessel, and it simultaneously handles two different blood circuits. Because of the unique structure of the placenta, the two different blood supplies don't mix. According to “Vascular Biology of the Placenta,” approximately 600 to 700 milliliters of blood travel through the placenta each minute once the baby is near term.
Family Education explains that after the baby is born, the vessels close automatically. The arteries close first, and the vein closes after about three or four minutes. Because the arteries close first, no blood leaves the baby and enters the placenta after birth, but some fresh blood can still reach the baby if necessary.