The maternal vein works with the maternal artery and tissue from both the mother and fetus to form the placenta. Throughout the stage of fetal development, maternal blood supplies the fetus with nutrients and oxygen, and carries away its waste. These substances diffuse between the maternal arteries and veins, and fetal blood through the placental membrane, and they are carried to and from the fetal body by the umbilical vein and artery within the umbilical cord. Consequently, the fetal blood and vascular system must be highly adapted to intrauterine life in special ways. Blood does not flow directly between mother and fetus, but because of the proximity of the tissues comprising the placental membrane, certain substances diffuse readily.