When to Start Iron Supplementation in Pregnancy
Start iron supplementation at the first prenatal visit in early pregnancy with 30 mg of elemental iron daily for all pregnant women, or alternatively screen with serum ferritin in early pregnancy and provide 40-60 mg daily starting by gestational week 10-18 for those at risk. 1
Timing of Initiation
The evidence strongly supports early initiation of iron supplementation:
- Begin supplementation in the first trimester or at the first prenatal visit rather than waiting until 20 weeks gestation. 1
- Early supplementation (before 20 weeks) is significantly more effective at preventing iron deficiency and iron deficiency anemia than late supplementation (at or after 20 weeks), regardless of the dose used. 2
- Women who start iron supplementation early in pregnancy maintain better iron stores throughout pregnancy and postpartum compared to those who start later. 3
- Early supplementation with doses around 100 mg/day improves maternal biochemical iron status without causing harmful hemoconcentration, regardless of pre-pregnancy iron levels. 3
Recommended Dosing Strategy
Standard Prophylactic Approach
- 30 mg of elemental iron daily starting in early pregnancy is the typical dose prescribed in prenatal vitamins. 1
- For populations at increased risk for iron deficiency anemia, prescribe 40-60 mg of elemental iron daily starting by gestational week 10-18. 1, 4
Evidence-Based Dose Selection
- 40 mg of ferrous iron daily starting at 18 weeks prevents iron deficiency in 90% of women and iron deficiency anemia in at least 95% during pregnancy and postpartum. 4
- Doses of 20 mg daily are insufficient, resulting in 50% iron deficiency at 32 weeks and 29% at 39 weeks. 4
- Doses above 40 mg (60-80 mg) provide no additional benefit in preventing iron deficiency or anemia compared to 40 mg. 4
Risk Stratification
High-Risk Populations Requiring Higher Doses (60-100 mg daily)
- Non-Hispanic Black and Mexican American women 1
- Women with parity ≥2 1
- Vegetarian or vegan diet 1, 5
- Women ≥40 years of age 5
- Diet lacking iron-rich foods 1
- Gastrointestinal disease or medications that decrease iron absorption (e.g., antacids) 1
- Short interval between pregnancies 1
- Women with pre-pregnancy serum ferritin <20 μg/L 3
Selective Screening Approach
- If using a selective approach, screen with serum ferritin in early pregnancy to identify women who need prophylactic iron. 6
- Women with iron reserves <500 mg (approximately 80% of fertile women) will require supplementation during pregnancy. 6
Clinical Rationale
Physiologic Iron Demands
- Iron absorption requirements increase from 0.8 mg/day in early pregnancy to 7.5 mg/day in late pregnancy. 6
- The Recommended Dietary Allowance for iron in pregnant women is 27 mg per day, which most women do not achieve through diet alone. 1
- Without supplementation, 20% of pregnant women develop iron deficiency anemia. 6
Benefits of Early Supplementation
- Early iron supplementation improves birth weight specifically in women who start pregnancy with iron deficiency (serum ferritin <20 μg/L). 3
- Children born to iron-supplemented mothers have higher serum ferritin levels than those born to non-supplemented mothers. 6
- Iron-treated pregnant women maintain higher hemoglobin levels and greater iron reserves both during pregnancy and postpartum. 6
Common Pitfalls to Avoid
- Do not wait until 20 weeks gestation to initiate supplementation, as this timing is too late to prevent iron deficiency in many women. 2
- Do not prescribe only 20 mg daily, as this dose is inadequate for prevention. 4
- Do not assume dietary intake alone is sufficient, as the majority of women have iron intake below the estimated allowance of 12-18 mg/day. 6
- Do not increase doses beyond 60 mg in an attempt to improve efficacy, as higher doses provide no additional benefit and may increase gastrointestinal side effects. 4