Definition of Anemia in Pregnancy
Anemia in pregnancy is defined by hemoglobin thresholds that vary by trimester: hemoglobin <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. 1, 2
Trimester-Specific Diagnostic Thresholds
The diagnostic criteria for anemia change throughout pregnancy due to physiologic hemodilution:
- First trimester: Hemoglobin <11.0 g/dL 1, 2, 3
- Second trimester: Hemoglobin <10.5 g/dL 1, 2, 3
- Third trimester: Hemoglobin <11.0 g/dL 1, 2, 3
These trimester-specific cutoffs reflect the progressive decline in hemoglobin concentration that occurs during the first and second trimesters as plasma volume expands disproportionately to red blood cell mass. 4
Alternative Definitions
The World Health Organization uses slightly different criteria, defining anemia in pregnancy as hemoglobin <110 g/L (11.0 g/dL) in the second and third trimesters. 5, 6 Some guidelines also reference hemoglobin <11.0 g/dL in late first trimester and <10.0 g/dL in second and third trimesters as lower limits. 7
Physiologic Context
Understanding the physiologic hemodilution of pregnancy is critical to avoid misdiagnosis. Hemoglobin and hematocrit naturally decline during the first and second trimesters because blood volume expands by approximately 35%, with plasma volume increasing more than red blood cell mass. 5, 4 Among pregnant women taking adequate iron supplementation, hemoglobin gradually rises during the third trimester toward prepregnancy levels. 5
Clinical Pitfalls to Avoid
Excessively high hemoglobin values (>15.0 g/dL) or hematocrit (>45%) during the second or third trimester should prompt evaluation for poor blood volume expansion, which is associated with adverse outcomes including preterm delivery and fetal growth restriction. 5, 4 This paradoxically indicates pathology rather than health.
The lower limit of normal should align with WHO-defined thresholds and be appropriate for the laboratory performing the test. 5 Using hemoglobin or hematocrit measurement alone is imprecise for determining true iron deficiency status due to the physiologic changes of pregnancy. 4
Severity Classification
When anemia is confirmed, severity can be categorized as:
- Mild anemia: Hemoglobin ≥10.0 g/dL 2
- Moderate anemia: Hemoglobin 7.0-9.9 g/dL 2
- Severe anemia: Hemoglobin <9.0 g/dL or hematocrit <27.0% (requires physician referral) 5, 1
Severe anemia with hemoglobin <6.0 g/dL is associated with poor pregnancy outcomes including prematurity, spontaneous abortion, low birth weight, and fetal death. 7