Iron Supplementation Should Not Be Withheld in Any Trimester
Iron supplementation is recommended throughout all trimesters of pregnancy, and there is no trimester in which iron should be categorically avoided. However, caution is warranted when hemoglobin exceeds 15.0 g/dL or hematocrit exceeds 45.0% in the second or third trimester, as this may indicate poor blood volume expansion rather than optimal iron status 1.
Understanding the Question's Premise
The premise that iron should be avoided in a specific trimester is not supported by current guidelines. Instead, the evidence reveals important nuances about iron status interpretation across pregnancy:
Physiologic Changes Across Trimesters
Hemoglobin and hematocrit naturally decline during the first and second trimesters due to physiologic blood volume expansion, which is a normal and healthy adaptation to pregnancy 1.
In the third trimester, hemoglobin and hematocrit gradually rise toward prepregnancy levels in women with adequate iron intake 1.
The prevalence of iron deficiency increases progressively from 6.9% in the first trimester to 29.5% in the third trimester, making continued supplementation important 1.
When to Exercise Caution (Not Avoidance)
High Hemoglobin/Hematocrit as a Warning Sign
Excessively high hemoglobin (>15.0 g/dL) or hematocrit (>45.0%), particularly in the second trimester, should prompt evaluation rather than continued routine supplementation 1, 2:
High hematocrit (≥43%) at 26-30 weeks gestation is associated with more than a twofold increased risk for preterm delivery 1.
Women with elevated hematocrit have a fourfold increased risk for fetal growth retardation compared to those with hematocrit of 33-36% 1.
High hemoglobin/hematocrit in the second or third trimester has been linked to hypertension, fetal death, preterm delivery, and low birthweight 1.
This elevated hemoglobin likely indicates poor blood volume expansion rather than desirable iron status and warrants clinical evaluation 1, 2.
Standard Supplementation Recommendations
Universal Prophylaxis
The CDC recommends 30 mg/day of elemental iron starting at the first prenatal visit for all pregnant women 2.
This prophylactic dose should continue throughout pregnancy unless hemoglobin becomes excessively elevated 2.
Treatment Doses
For confirmed anemia, 60-120 mg/day of elemental iron is recommended 2, 3.
Once hemoglobin normalizes for gestational age, reduce to maintenance dose of 30 mg/day 2.
Clinical Pitfalls to Avoid
Common Misinterpretation
Do not interpret low hemoglobin in the second trimester as necessarily pathologic—physiologic hemodilution is expected and healthy 1.
Do not assume high hemoglobin in late pregnancy indicates excellent iron status—it may signal pathologic failure of blood volume expansion 1.
Monitoring Strategy
Screen at first prenatal visit and again at 24-28 weeks gestation 2.
Reassess hemoglobin/hematocrit after 4 weeks of treatment for anemia 2.
Expected response is ≥1 g/dL increase in hemoglobin or ≥3% increase in hematocrit 2.