<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> <P> The core concept behind fetal circulation is that fetal hemoglobin (HbF) has a higher affinity for oxygen than does adult hemoglobin, which allows a diffusion of oxygen from the mother's circulatory system to the fetus . </P> <P> It is the fetal heart and not the mother's heart that builds up the fetal blood pressure to drive its blood through the fetal circulation . </P> <P> Intracardiac pressure remains identical between the right and left ventricles of the human fetus . </P>

2 major heart changes that occur between fetal and postnatal circulations