At what age does physiologic anemia (Physiologic Anemia of Infancy) typically occur?

Onset of Physiologic Anemia of Infancy

Physiologic anemia of infancy typically occurs at approximately 2-3 months of age in term infants, with hemoglobin levels reaching their nadir (lowest point) around 6-12 weeks of life. 1

Physiologic Mechanism and Timeline

During the first 6 weeks of life, hemoglobin levels fall from an average of 170 g/L at birth to approximately 120 g/L due to temporary suppression of erythropoiesis after transition from the relatively hypoxic intrauterine environment to the oxygen-rich atmosphere outside. 1 This physiologic process involves:

• Blood volume expansion as the infant grows 1
• Breakdown of fetal hemoglobin during the switch to adult-type hemoglobin 1
• Recirculation of iron from senescent erythrocytes to iron stores, which actually increase in size during this period 1

Critical Distinction: Physiologic Anemia vs. Iron Deficiency Anemia

It is essential to understand that physiologic anemia of infancy (occurring at 2-3 months) is completely different from iron deficiency anemia, which generally does not occur until approximately 9 months of age in full-term infants. 1, 2

Why This Timing Difference Matters:

• Iron stores of full-term infants can meet requirements until ages 4-6 months, rendering healthy term infants initially almost independent of additional external iron until they have doubled their birth weight 1
• During the first 6 months following physiologic anemia, as the infant grows and expands blood volume, iron is transferred back from stores to the blood compartment 1
• Iron deficiency anemia develops later (around 9 months) when rapid growth coincides with inadequate dietary iron intake 1, 2

Preterm Infants: Earlier Onset

Preterm infants experience physiologic anemia earlier and more severely than term infants, with iron stores often depleted by ages 2-3 months. 1, 2 This occurs because:

• Preterm and low-birthweight infants are born with lower iron stores 1
• They grow faster during infancy, accelerating depletion 1
• They reach their hemoglobin nadir sooner and at a lower level than term infants 3

Clinical Implications

The physiologic anemia at 2-3 months is a normal, self-limited process that does not require intervention in healthy term infants. 1 However:

• Dietary iron requirements during the first 6 months for exclusively breastfed term neonates are extremely low (approximately 0.27 mg/day) 1
• Vigilance for pathologic iron deficiency anemia should increase after 6 months of age, with peak risk at 9-18 months 1, 2