In which trimester of pregnancy is iron supplementation typically started?

Iron Supplementation in Pregnancy: Timing and Trimester

Iron supplementation should be started at the first prenatal visit, regardless of trimester, with low-dose oral iron (30 mg/day of elemental iron) for all pregnant women. 1, 2

Rationale for Early Initiation

The CDC guidelines explicitly recommend starting oral iron supplementation at the first prenatal visit as a primary prevention strategy, not waiting for a specific trimester. 1 This approach is based on the understanding that:

• Iron demands increase dramatically in the second and third trimesters (threefold increase to approximately 5.0 mg iron/day), making it critical to build stores early. 1
• Most pregnant women cannot maintain adequate iron stores through diet alone, particularly during the second and third trimesters, even with increased iron absorption during pregnancy. 1
• Only one-fourth of women of childbearing age meet the recommended dietary allowance for iron through diet before pregnancy, placing them at high risk for depletion. 1

Supplementation Throughout Pregnancy

All trimesters require attention, but the physiologic demands differ:

• First trimester: Iron requirements are relatively lower (approximately 0.8 mg/day) due to cessation of menstruation, but supplementation should still begin to build stores. 3
• Second trimester: Iron requirements rise to 4-5 mg/day, and the prevalence of anemia increases from 9% in the first trimester to 14% in the second trimester among low-income pregnant women. 1, 3
• Third trimester: Iron requirements exceed 6 mg/day, and anemia prevalence reaches 37% in high-risk populations. 1, 3

Dosing Strategy

The standard preventive dose is 30 mg/day of elemental iron, which should be initiated at the first prenatal visit. 1, 2 This low-dose regimen:

• Is as effective as higher doses (60-120 mg/day) in preventing iron-deficiency anemia. 1
• Reduces unwanted gastrointestinal side effects (constipation, nausea, vomiting) that compromise compliance. 1
• Should be continued throughout pregnancy and the postpartum period. 2, 4

Treatment Doses for Diagnosed Anemia

If anemia is diagnosed at any point during pregnancy, increase the dose to 60-120 mg/day of elemental iron. 1, 2 After hemoglobin normalizes for gestational age, reduce back to the maintenance dose of 30 mg/day. 1, 2

Common Pitfalls to Avoid

• Do not wait until the second or third trimester to start supplementation—the CDC explicitly recommends starting at the first prenatal visit, which may occur in any trimester. 1, 2
• Do not assume dietary intake alone is sufficient—even optimal diets cannot meet the iron requirements of later pregnancy without pre-existing stores of ≥300 mg. 3
• Screen for anemia at the first prenatal visit and again at 24-28 weeks gestation using hemoglobin or hematocrit testing. 2

Evidence on Early vs. Late Supplementation

Research comparing early versus late supplementation with moderate doses (40-43 mg/day) found no significant difference in preventing iron deficiency when supplementation was started before versus after 20 weeks of gestation. 5 However, this does not negate the guideline recommendation to start at the first prenatal visit, as building iron stores early remains the standard of care to prevent depletion during peak demand periods. 1, 2