What are the hemoglobin levels that define anemia in pregnancy by trimester?

Hemoglobin Ranges Defining Anemia in Pregnancy by Trimester

According to the Centers for Disease Control and Prevention and the American College of Obstetricians and Gynecologists, anemia in pregnancy is defined as a hemoglobin level less than 11.0 g/dL in the first trimester, less than 10.5 g/dL in the second trimester, and less than 11.0 g/dL in the third trimester. 1, 2

Trimester-Specific Hemoglobin Cutoffs for Anemia

• First Trimester: Hemoglobin < 11.0 g/dL 3, 1, 2
• Second Trimester: Hemoglobin < 10.5 g/dL 3, 1, 2
• Third Trimester: Hemoglobin < 11.0 g/dL 3, 1, 2

Physiological Basis for Trimester-Specific Values

• During pregnancy, hemoglobin concentration naturally decreases during the first and second trimesters due to expanding blood volume (hemodilution) 3
• In women with adequate iron intake, hemoglobin concentration gradually rises during the third trimester toward pre-pregnancy levels 3, 1
• In women who do not take iron supplements, hemoglobin concentration remains low throughout the third trimester 3

Classification of Anemia Severity in Pregnancy

• Mild Anemia:
  • First trimester: Hemoglobin 10.0-10.9 g/dL 1, 2
  • Second/Third trimester: Hemoglobin 10.0-10.4 g/dL 1
• Moderate Anemia: Hemoglobin 7.0-9.9 g/dL (all trimesters) 1, 2, 4
• Severe Anemia: Hemoglobin < 7.0 g/dL (all trimesters) 1, 2, 4

Factors Affecting Hemoglobin Values in Pregnancy

• Altitude: Hemoglobin values should be adjusted upward for women living at high altitudes (>3,000 ft) 3
  • For example, add 0.5 g/dL at 1,500 meters, 0.8 g/dL at 2,000 meters 3
• Smoking: Smoking increases hemoglobin values 3
  • Add 0.3-0.7 g/dL depending on smoking intensity 3
• Ethnicity: Black women may have lower normal hemoglobin values than white women 3, 5

Clinical Implications and Monitoring

• Anemia prevalence increases with advancing gestational age, with highest rates in the third trimester 6, 7
• Severe anemia (Hb < 7.0 g/dL) is associated with significantly increased risks of adverse maternal and fetal outcomes including:
  • Placental abruption (3.35 times higher risk) 4
  • Preterm birth (1.36 times higher risk) 4
  • Severe postpartum hemorrhage (15.65 times higher risk) 4
  • Maternal ICU admission (2.88 times higher risk) 4
  • Fetal malformation (1.62 times higher risk) 4
• Screening for anemia should occur at first prenatal visit and again at 24-28 weeks gestation 2

Warning Signs for High Hemoglobin

• Hemoglobin > 15.0 g/dL or hematocrit > 45.0%, particularly in the second trimester, may indicate poor blood volume expansion 3
• High hemoglobin/hematocrit has been associated with hypertension and poor pregnancy outcomes including fetal growth restriction, fetal death, preterm delivery, and low birth weight 3
• Women with hematocrit ≥ 43% at 26-30 weeks' gestation have more than twofold increased risk for preterm delivery and fourfold increased risk for fetal growth restriction 3

Treatment Thresholds

• For mild anemia (Hb 10.0-10.9 g/dL), a therapeutic trial of oral iron (60-120 mg/day) is recommended 1, 2
• For moderate to severe anemia (Hb < 10.0 g/dL), further evaluation and possible specialist referral is warranted 1, 2
• For severe anemia (Hb < 7.0 g/dL), urgent blood transfusion should be considered, especially with clinical symptoms 1, 4