When people think of blood, they think of it being basically the same in all of humans, with only changes in blood type or other minor differences. This is not true. A quick lesson is that in blood is hemoglobin. The hemoglobin, which binds oxygen, in humans shortly birth is divided into four subunits with two of them being alpha and two of them being beta subunits. While still in the fetal stage, the beta subunits are actually gamma subunits. The reason of these changes in from the fact that the histidine, an amino acid, is replaced with a serine, another amino acid, at the 143 position of the beta subunit. Over the first few months after birth, the fetal hemoglobin changes to the adult hemoglobin in the blood.

The histidine to serine alteration changes the interactions of the hemoglobin with oxygen. This causes the blood in fetus to have a higher affinity for oxygen then the mother. This is important because a lot of the oxygen from the blood is being used by the mother for basic living functions. If the fetus did not have a higher affinity then too little oxygen would be received by fetus and it would die.

This greater affinity for oxygen is explained by fetal hemoglobin's interaction with 2,3-bisphosphoglycerate (2,3-BPG). 2,3-BPG lowers the affinity for oxygen in the blood. While this is important for living everyday, it is not good for the fetus, because the fetus needs a higher amount of oxygen affinity. Since 2,3-BPG is a highly negative molecule, it needs the positive charge from the histidine to bind to the hemoglobin. Since the histidine has been replaced by a neutral charged serine, the 2,3-BPG does not bind resulting in the higher affinity of oxygen.