<P> In newborns, fetal hemoglobin is nearly completely replaced by adult hemoglobin by approximately 6 months postnatally, except in a few thalassemia cases in which there may be a delay in cessation of HbF production until 3--5 years of age . In adults, fetal hemoglobin production can be reactivated pharmacologically, which is useful in the treatment of diseases such as sickle - cell disease . </P> <P> Oxygenated blood is delivered to the fetus via the umbilical vein from the placenta, which is anchored to the wall of the mother's uterus . The chorion acts as a barrier between the maternal and fetal circulation so that there is no admixture of maternal and fetal blood . Blood in the maternal circulation is delivered via open ended arterioles to the intervillous space of the chorionic plate, where it bathes the chorionic villi that carry umbilical capillary beds, thereby allowing gas exchange to occur between the maternal and fetal circulation . Deoxygenated maternal blood drains into open ended intervillous venules to return to maternal circulation . Due to the admixture of oxygenated and deoxygenated blood, maternal blood in the intervillous space is lower in oxygen than arterial blood . As such, fetal hemoglobin must be able to bind oxygen with greater affinity than adult hemoglobin in order to compensate for the relatively lower oxygen tension of the maternal blood supplying the chorion . </P> <P> Fetal hemoglobin's affinity for oxygen is substantially greater than that of adult hemoglobin . Notably, the P50 value for fetal hemoglobin is lower than adult hemoglobin (i.e., the partial pressure of oxygen at which the protein is 50% saturated; lower values indicate greater affinity). The P50 of fetal hemoglobin is roughly 19 mmHg, whereas adult hemoglobin is approximately 26.8 mmHg . As a result, the "oxygen saturation curve", which plots percent saturation vs. pO, is left - shifted for fetal hemoglobin as compared to adult hemoglobin . </P> <P> This greater affinity for oxygen is explained by the lack of fetal hemoglobin's interaction with 2, 3 - bisphosphoglycerate (2, 3 - BPG or 2, 3 - DPG). In adult red blood cells, this substance decreases the affinity of hemoglobin for oxygen . 2, 3 - BPG is also present in fetal red blood cells, but interacts less efficiently with fetal hemoglobin than adult hemoglobin . This is due to a change in a single amino acid (residue 143) found in the 2, 3 - BPG' binding pocket': from histidine to serine, which gives rise to the greater oxygen affinity . </P>

Oxygen dissociation curve of maternal and fetal blood