Which statement best describes HbF percentage changes with increasing postnatal age in preterm infants?

• A. HbF Remains 100%
• B. HbF Decreases as Age Increases
• C. HbF Increases as Age Increases
• D. HbF Is Not Present at Birth

Answer: (B)

After birth, the newborn’s blood gradually shifts from fetal hemoglobin (HbF), which has gamma chains and a high oxygen affinity, to adult hemoglobin (HbA) with beta chains. This switch happens over the first weeks to months of life. In preterm infants, the switch to HbA occurs but tends to proceed more slowly because of immaturity of the erythropoietic system. Despite that slower pace, the percentage of HbF falls as postnatal age increases. That decline in HbF with advancing age is why the statement that HbF decreases as age increases is the best description.

HbF remaining at 100% would be incorrect because HbA begins to appear right after birth. HbF increasing with age contradicts the known postnatal switch to HbA. HbF not present at birth is false because HbF is present in high amounts at birth in all newborns, especially preterm infants.