Hemodilution is the Typical Result of Blood Volume Expansion During Pregnancy
The correct answer is C: hemodilution is typical during pregnancy due to a disproportionate increase in plasma volume compared to red blood cell mass. 1
Physiological Mechanism of Hemodilution
During normal pregnancy, plasma volume increases dramatically by approximately 40-50% (averaging 1250 mL), while red cell mass increases by only 20-30% (250-450 mL depending on iron supplementation). 1, 2 This differential expansion creates a physiologic hemodilution that is both normal and expected.
The plasma volume expansion begins after the first trimester and reaches its maximum at approximately 34-36 weeks of gestation. 2 This results in:
- A decline in hemoglobin concentration and hematocrit during the first and second trimesters 1
- Venous hematocrit dropping from a non-pregnant average of approximately 40% to about 33% during the last trimester 2
- Mean platelet counts decreasing by approximately 17% during pregnancy due to the dilutional effect 1
Why the Other Options Are Incorrect
Option A (decrease in red blood cell volume) is false - Red blood cell mass actually increases during pregnancy, not decreases, rising by 18-25% depending on iron supplementation status. 2 The apparent decrease in red cell concentration is due to dilution, not an actual reduction in total red cell volume.
Option B (reduced leukocytosis) is incorrect - Pregnancy does not reduce the occurrence of leukocytosis. In fact, leukocytosis can occur as a normal physiologic response during pregnancy and is recognized as a risk factor in certain conditions like myeloproliferative neoplasms. 1
Option D (systemic vascular resistance is a danger) is misleading - While systemic vascular resistance decreases during pregnancy (not increases), this is a normal adaptive response mediated by progesterone, estrogen, and nitric oxide. 1 The decrease in systemic vascular resistance is physiologic and compensatory, not dangerous in normal pregnancy.
Clinical Significance of Hemodilution
Importantly, inadequate plasma volume expansion (failure to achieve normal hemodilution) is actually associated with poor pregnancy outcomes. 1 A hemoglobin concentration or hematocrit that remains elevated (>3 standard deviations above the mean, such as Hb >15.0 g/dL or Hct >45%) in the second trimester likely indicates poor blood volume expansion and has been associated with:
- Hypertension 1
- Fetal growth retardation 1
- Preterm delivery 1
- Increased risk of stillbirth (when Hb >14.6 g/dL) 1
Women with a hematocrit ≥43% at 26-30 weeks' gestation have more than a twofold increased risk for preterm delivery and a fourfold increased risk for fetal growth retardation compared to women with Hct of 33-36%. 1
Common Pitfall to Avoid
The most important clinical pitfall is misinterpreting the physiologic hemodilution of pregnancy as pathologic anemia requiring aggressive treatment. 1 While true iron deficiency anemia does occur and requires treatment, the expected decline in hemoglobin and hematocrit due to normal plasma volume expansion should not be over-corrected, as failure to achieve adequate hemodilution is associated with worse outcomes. 1