To determine when synthesis of hemoglobin A (composed of 2 α and 2 β chains and the hemoglobin predominant in the human adult) begins in the fetus, hemoglobin synthesis was measured in fetuses of various gestational ages. Synthesis of hemoglobin A by reticulocytes obtained from 42 embryos and fetuses 3.5 to 20.0 cm in crown-to-rump length accounted for 4.3 to 13.0 per cent of total hemoglobin synthesis, and varied directly with fetal crown-to-rump length. Since the smallest embryo was estimated to be 55 days old, β-chain synthesis has begun by that gestational age. Furthermore, since β thalassemia is characterized by decreased β-chain synthesis, the accurate diagnosis of this condition in fetuses depends on knowledge of the values for hemoglobin A synthesis in normal fetuses of variousgestational ages. (N Engl J Med 289:58–62, 1973).
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