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Difficulty: EasyNormal Pregnancy and Prenatal Care

A 27-year-old G1P0 at 28 weeks gestation arrives at the obstetric clinic for a scheduled prenatal visit. She works as a teacher and mentions mild fatigue by the late afternoon, but otherwise feels healthy. She is fully compliant with her daily prenatal vitamin. Vital signs demonstrate a blood pressure of 112/68 mm Hg and a heart rate of 84/min. Physical examination confirms a fundal height of 28 cm, and the fetal heart rate is reassuring at 150/min. A routine complete blood count is obtained and shows the following results:

Hemoglobin: 10.8 g/dL
Hematocrit: 33%
Mean corpuscular volume (MCV): 88 fL
Platelet count: 215,000/mm³

Which of the following physiologic mechanisms best explains this patient's hematologic profile?

  1. Disproportionate increase in plasma volume relative to red blood cell massAnswer
  2. B
    Depletion of maternal iron stores secondary to fetal utilization
  3. C
    Decreased maternal erythropoietin production by the kidneys
  4. D
    Inadequate dietary folate intake exceeding maternal reserves
  5. E
    Increased destruction of erythrocytes in the placental bed

Answer

The hematologic profile is a result of physiologic hemodilution, driven by a disproportionate increase in plasma volume compared to red blood cell mass.
This patient's hemoglobin of 10.8 g/dL and normal mean corpuscular volume (MCV) at 28 weeks gestation represent normal physiologic dilutional anemia. During an uncomplicated pregnancy, maternal plasma volume expands by approximately 40-50% to accommodate fetal demands and protect against expected peripartum blood loss. Simultaneously, red blood cell (RBC) mass increases by only about 20-30%. Because the expansion in plasma volume is substantially larger than the increase in RBC mass, the hematocrit and hemoglobin concentration decrease relative to the non-pregnant state. A hemoglobin level of greater than 10.5 g/dL is considered normal in the second trimester, and > 11.0 g/dL is normal in the first and third trimesters. No further workup or treatment beyond routine prenatal care is indicated.

Step-by-Step Solution

1
Interpret the laboratory findings.
The patient has a hemoglobin of 10.8 g/dL and a normal MCV of 88 fL at 28 weeks gestation.
Establishing whether the laboratory values represent a pathological state or a normal physiologic change of pregnancy is the first required step.
2
Compare findings to pregnancy-specific normal ranges.
A hemoglobin level above 10.5 g/dL in the second trimester (or > 11.0 g/dL in the first and third trimesters) is considered normal.
Diagnostic criteria for anemia differ in pregnant patients due to expected adaptations.
3
Identify the underlying physiologic mechanism.
The correct mechanism is hemodilution caused by plasma volume expanding significantly more than red blood cell mass.
Understanding the physiologic adaptations to pregnancy allows differentiation from true pathologic anemia.

Key Concept

Physiologic Anemia of Pregnancy
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Topics

Normal Pregnancy and Prenatal Care

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