What are the target hemoglobin (Hb) and iron (Fe) levels in pregnant women?

Target Hemoglobin and Iron Levels in Pregnant Women

In pregnant women, target hemoglobin levels to define anemia are <11.0 g/dL in the first trimester, <10.5 g/dL in the second trimester, and <11.0 g/dL in the third trimester, with serum ferritin levels of 12.0-30.0 µg/L considered adequate when supplemented. 1, 2

Trimester-Specific Hemoglobin Targets

First Trimester (Weeks 1-13)

• Anemia threshold: Hemoglobin <11.0 g/dL 1, 2
• Mild anemia is classified as hemoglobin 10.0-10.9 g/dL 1
• The 5th percentile for hemoglobin in iron-supplemented women is 111 g/L (11.1 g/dL) 3

Second Trimester (Weeks 14-27)

• Anemia threshold: Hemoglobin <10.5 g/dL 1, 2
• This lower threshold reflects physiologic hemodilution from expanding blood volume 4
• Mild anemia is classified as hemoglobin 10.0-10.4 g/dL 1
• The 5th percentile for hemoglobin in iron-supplemented women ranges from 106-109 g/L (10.6-10.9 g/dL) depending on the specific week 3

Third Trimester (Weeks 28-40)

• Anemia threshold: Hemoglobin <11.0 g/dL 1, 2
• Hemoglobin gradually rises toward pre-pregnancy levels in women taking iron supplements 4
• The 5th percentile for hemoglobin in iron-supplemented women ranges from 105-110 g/L (10.5-11.0 g/dL) 3

Target Iron (Ferritin) Levels

Adequate Iron Status During Pregnancy

• Target serum ferritin at term/delivery: 12.0-30.0 µg/L in supplemented women 4
• Iron deficiency is defined as serum ferritin <12.0 ng/mL 5
• Women receiving 20-120 mg daily iron supplementation achieved ferritin levels of 12.0-30.0 µg/L at delivery 4

Important Caveats About Ferritin Measurement

• Serum ferritin has limited utility in late pregnancy because it decreases despite adequate bone marrow iron stores 4
• Ferritin is an acute phase reactant and increases during inflammation, potentially masking iron deficiency 4
• Hemoglobin alone has poor diagnostic accuracy for iron deficiency in pregnancy (sensitivity 60.7%, specificity 44.3%) 5

Clinical Algorithm for Iron Supplementation

Universal Prophylaxis

• Start 30 mg/day oral iron at the first prenatal visit for all pregnant women 4
• Continue throughout pregnancy to prevent iron deficiency anemia 4

Treatment of Confirmed Anemia

• Prescribe 60-120 mg/day oral iron when anemia is confirmed 4
• Recheck hemoglobin after 4 weeks of treatment 4
• Expect hemoglobin to increase by ≥1 g/dL or hematocrit by ≥3% if treatment is effective 4
• Reduce dose to 30 mg/day once hemoglobin normalizes for gestational age 4

When to Escalate Evaluation

• Hemoglobin <9.0 g/dL or hematocrit <27.0% requires physician referral for further evaluation 4
• No response to iron therapy after 4 weeks (with confirmed compliance) warrants additional testing including MCV, RDW, and serum ferritin 4
• Consider thalassemia minor or sickle cell trait in women of African, Mediterranean, or Southeast Asian ancestry with mild anemia unresponsive to iron 4

Warning Signs: Dangerously High Hemoglobin

Upper Limit Concerns

• Hemoglobin >15.0 g/dL or hematocrit >45.0% during second or third trimester indicates poor blood volume expansion 4, 1
• This paradoxically increases risk for preterm delivery (>2-fold), fetal growth restriction (>4-fold), fetal death, and low birth weight 1
• Evaluate for pregnancy complications related to inadequate plasma volume expansion 4

Special Adjustments Required

Altitude Considerations

• Hemoglobin values must be adjusted upward for women living at altitudes >3,000 feet 1
• Failure to adjust leads to underdiagnosis of anemia 1

Smoking Adjustments

• Smoking increases hemoglobin by 0.3-0.7 g/dL depending on intensity 4, 1
• Adjust anemia thresholds upward by 0.3 g/dL for all smokers, 0.5 g/dL for 1-2 packs/day, and 0.7 g/dL for >2 packs/day 4

Racial Considerations

• Black women may have hemoglobin values 0.5-0.9 g/dL lower than white women as normal variation 4
• This represents physiologic variation, not iron deficiency 1

Screening Recommendations

Screen all pregnant women at the first prenatal visit and again at 24-28 weeks gestation with complete blood count 2. The evidence supporting universal screening is limited, but the USPSTF recommends screening given the high prevalence of iron deficiency (ranging from 6.9% in first trimester to 29.5% in third trimester) 4.