 The test is based on differences between maternal and fetal hemoglobin.Maternal blood contains adult hemoglobin composed of two alpha and two beta subunits (aka hemoglobin A or HbA; i.e., normal adult hemoglobin).Fetal blood contains fetal hemoglobin composed of two alpha and two gamma subunits (aka hemoglobin F or HbF; i.e., normal fetal hemoglobin).