Definition of Anemia in Pregnancy
Anemia in pregnancy is defined as a hemoglobin concentration less than 11.0 g/dL in the first trimester and less than 10.5 g/dL in the second and third trimesters. 1
Diagnostic Criteria
The definition of anemia in pregnancy is based on trimester-specific hemoglobin cutoffs:
- First trimester: Hemoglobin < 11.0 g/dL
- Second trimester: Hemoglobin < 10.5 g/dL
- Third trimester: Hemoglobin < 10.5 g/dL
These values represent less than the 5th percentile of the distribution of hemoglobin concentration in a healthy reference population of pregnant women 2. This case definition has been shown to correctly identify 37% of women of childbearing age who were iron deficient (sensitivity) and to correctly classify 93% of women as not having iron deficiency (specificity) 2.
Physiological Basis
The trimester-specific definition accounts for normal physiological changes during pregnancy:
- During the first and second trimesters, hemoglobin concentration and hematocrit decline due to expanding blood volume 2
- In women with adequate iron intake, hemoglobin concentration gradually rises during the third trimester toward pre-pregnancy levels 2
- In women who do not take iron supplements, hemoglobin concentration remains low in the third trimester 2
Severity Classification
Anemia severity can be categorized based on hemoglobin levels:
- Mild anemia: Hemoglobin 10.0-10.9 g/dL (first trimester) or 10.0-10.4 g/dL (second/third trimesters)
- Moderate anemia: Hemoglobin 7.0-9.9 g/dL
- Severe anemia: Hemoglobin < 7.0 g/dL 1, 3
Important Considerations
- Adjustment factors: Hemoglobin cutoffs should be adjusted for altitude, smoking status, and race 2
- High hemoglobin concerns: A hemoglobin concentration > 15.0 g/dL or hematocrit > 45.0%, particularly in the second trimester, may indicate poor blood volume expansion and is associated with adverse pregnancy outcomes including hypertension, fetal growth retardation, preterm delivery, and low birthweight 2
- Prevalence: The prevalence of anemia increases throughout pregnancy, ranging from 1.8% in the first trimester to as high as 27.4% in the third trimester 1, 4
Clinical Implications
Anemia in pregnancy, particularly when severe (Hb < 6 g/dL), is associated with significant adverse maternal and fetal outcomes:
- Preterm birth
- Low birth weight
- Small-for-gestational-age infants
- Stillbirth
- Perinatal mortality
- Postpartum hemorrhage
- Increased need for blood transfusion 1
Screening Recommendations
The Centers for Disease Control and Prevention and the American College of Obstetricians and Gynecologists recommend:
- Routine screening for anemia at the first prenatal visit
- Repeat screening at 24-28 weeks gestation 1, 5
Understanding the correct definition of anemia in pregnancy is crucial for proper diagnosis and management, as both untreated anemia and excessive iron supplementation can lead to adverse outcomes.