In humans and other mammals, the developing embryo and fetus express different forms of hemoglobin than does the mother. The oxygen affinities of fetal hemoglobin are considerably greater than that of maternal hemoglobin. This phenomenon fits with the fact that fetal hemoglobin must be oxygenated in the placenta, where the pO 2 is lower than it is in the lungs. The higher affinity (lower P 50) of fetal hemoglobin is due to its lower affinity for BPG. Because BPG binding and O 2 binding interfere with each other, reduced affinity for the former means increased affinity for the latter. Fetal hemoglobin is replaced by the mature form in human infants by about six months of age.