Hemoglobin F is the hemoglobin found in the mammalian fetus and hemoglobin A is the normal hemoglobin. Hemoglobin F has higher affinity for oxygen. The fetus needs hemoglobin capable of extracting oxygen from the mother’s circulation. Therefore the fetus uses hemoglobin F since it has higher affinity for oxygen than the mother’s hemoglobin. The Circulatory System - Image Diversity: hemoglobin F