A 27-year-old nulliparous woman at 24 weeks gestation comes to the clinic for a routine prenatal visit. She feels well and has no significant complaints. Her medical history is unremarkable, and she takes a daily prenatal vitamin. Vital signs are temperature 37.0°C (98.6°F), blood pressure 110/70 mmHg, pulse 82/min, and respirations 16/min. Laboratory results are as follows:
| Test | Result |
|---|---|
| Hemoglobin | 11.2 g/dL |
| Hematocrit | 33% |
| Mean corpuscular volume | 88 fL |
Her pre-pregnancy hemoglobin was 13.5 g/dL. Which of the following mechanisms best explains this patient's current laboratory findings?
- A greater increase in plasma volume relative to the increase in red blood cell massAnswer
- BDecreased gastrointestinal absorption of dietary iron
- CAccelerated destruction of erythrocytes in the maternal spleen
- DProgesterone-mediated suppression of bone marrow erythropoiesis
- EDepletion of maternal red blood cells from transfer to the fetal circulation
Answer
A greater increase in plasma volume relative to the increase in red blood cell mass
The correct answer accurately describes the mechanism of physiologic anemia of pregnancy. During pregnancy, maternal blood volume increases to meet the metabolic demands of the fetus and protect against delivery-related blood loss. Both plasma volume and red blood cell (RBC) mass increase. However, the plasma volume expands by approximately 40-50%, whereas the RBC mass only increases by 20-30%. This disproportionate expansion results in hemodilution, manifesting as a mild decrease in hemoglobin concentration and hematocrit, despite an overall increase in total oxygen-carrying capacity. This is a normal, expected finding.
Step-by-Step Solution
Key Concept
During pregnancy, plasma volume increases to a greater extent than red blood cell mass, resulting in a physiologic dilutional anemia.
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