CBC Findings in Physiologic Anemia of Pregnancy
In physiologic anemia of pregnancy, the CBC shows a decreased hemoglobin and hematocrit with a normal or mildly decreased mean corpuscular volume (MCV), reflecting hemodilution from disproportionate plasma volume expansion rather than true pathologic anemia. 1, 2
Hemoglobin and Hematocrit Values
Trimester-specific hemoglobin thresholds distinguish physiologic hemodilution from pathologic anemia:
- First trimester: Hemoglobin remains relatively preserved, with values ≥11.0 g/dL considered normal 2, 3
- Second trimester: The nadir occurs, with hemoglobin ≥10.5 g/dL representing normal physiologic hemodilution 2, 3
- Third trimester: Hemoglobin ≥11.0 g/dL is normal, though values may remain lower without iron supplementation 2, 4
The hematocrit follows a parallel decline, with the greatest dilutional effect occurring during the first and second trimesters when plasma volume expands more rapidly than red cell mass 1, 2.
Mechanism of CBC Changes
Plasma volume increases by approximately 40-50% (roughly 1250 mL) while red cell mass increases only 20-30% (250-450 mL), creating hemodilution that begins within the first 8-12 weeks of gestation 1, 2. This disproportionate expansion produces:
- Progressive decline in hemoglobin concentration starting in the first trimester 2
- Lowest hemoglobin values at 24-28 weeks gestation 2
- Gradual rise toward pre-pregnancy levels in the third trimester if iron supplementation is provided 1, 2
Mean Corpuscular Volume (MCV)
In pure physiologic anemia, the MCV remains normal or shows only mild reduction 3. This distinguishes it from:
- Iron deficiency anemia: Very low MCV with microcytic indices 3
- Folate deficiency: Elevated MCV with macrocytic changes 3, 5
Red Blood Cell Count
The absolute red blood cell count is increased in physiologic anemia, reflecting the 20-30% expansion in red cell mass 1. The anemia results from dilution, not decreased red cell production.
Critical Pitfalls to Avoid
Hemoglobin >14.6 g/dL or hematocrit >45% in early pregnancy signals inadequate plasma volume expansion, not "good iron stores," and is associated with increased risk of stillbirth, preeclampsia, fetal growth restriction, and preterm delivery 2, 4. This paradoxically represents pathology, not health.
Relying solely on hemoglobin/hematocrit to diagnose iron deficiency is unreliable because normal physiologic hemodilution can mask true iron deficiency 1. When anemia is confirmed (hemoglobin below trimester-specific thresholds), further evaluation with serum ferritin or a therapeutic trial of iron supplementation (60-120 mg elemental iron daily) is warranted 4, 3.
When to Suspect Pathologic Anemia
Investigate beyond physiologic anemia when:
- Very low MCV suggests iron deficiency or thalassemia trait 3
- Macrocytic indices suggest folate or B12 deficiency 3
- Hemoglobin <10.0 g/dL warrants diagnostic workup regardless of trimester 3
- Hemoglobin <9.0 g/dL requires referral to a physician experienced in pregnancy anemia management 4
- Failure to respond to 4 weeks of oral iron (expected rise ≥1 g/dL) necessitates evaluation for hemoglobinopathies, especially in high-risk ethnic populations 4, 3