The haemoglobin of human foetus (HbF) has 4 Heme group and 1 globin. In HbF the globin protein is made up of 2α and 2β polypeptide chains. Each α has 141 amino acids and each β-has 146 amino acids but 37 amino acids in each β are different from that of HbA. It has higher affinity for O2 even at low PO2 and the oxygen −Hb dissociation curve remains towards the left of normal.