<Tr> <Td> intra-hepatic portion of the fetal left umbilical vein (ductus venosus) </Td> <Td> ligamentum venosum </Td> </Tr> <Tr> <Td> proximal portions of the fetal left and right umbilical arteries </Td> <Td> umbilical branches of the internal iliac arteries </Td> </Tr> <Tr> <Td> distal portions of the fetal left and right umbilical arteries </Td> <Td> medial umbilical ligaments </Td> </Tr> <P> In addition to differences in circulation, the developing fetus also employs a different type of oxygen transport molecule in its hemoglobin from that when it is born and breathing its own oxygen . Fetal hemoglobin enhances the fetus' ability to draw oxygen from the placenta . Its oxygen - hemoglobin dissociation curve is shifted to the left, meaning that it is able to absorb oxygen at lower concentrations than adult hemoglobin . This enables fetal hemoglobin to absorb oxygen from adult hemoglobin in the placenta, where the oxygen pressure is lower than at the lungs . Until around six months' old, the human infant's hemoglobin molecule is made up of two alpha and two gamma chains (2α2γ). The gamma chains are gradually replaced by beta chains until the molecule becomes hemoglobin A with its two alpha and two beta chains (2α2β). </P>

When does blood first appear in a fetus