A 26-year-old woman, gravida 1, para 0, presents to the clinic for a scheduled prenatal evaluation at 26 weeks gestation. She has no complaints other than mild, intermittent fatigue. The patient is taking a daily prenatal vitamin containing iron and folic acid. Her vital signs are temperature 36.8°C (98.2°F), blood pressure 110/70 mm Hg, heart rate 82/min, and respiratory rate 14/min. Fetal heart tones are 145/min.
Laboratory studies obtained today show the following:
| Laboratory Study | Result | Reference Range (Nonpregnant) |
|---|---|---|
| Hemoglobin | 10.7 g/dL | 12.0 - 16.0 g/dL |
| Hematocrit | 32% | 36% - 46% |
| Mean corpuscular volume | 89 fL | 80 - 100 fL |
| Leukocyte count | 9,500/mm³ | 4,500 - 11,000/mm³ |
| Platelet count | 210,000/mm³ | 150,000 - 400,000/mm³ |
At her initial visit at 9 weeks gestation, her hemoglobin was 12.8 g/dL. Which of the following is the most likely primary mechanism responsible for this patient's current hematologic findings?
- Disproportionate increase in plasma volume relative to erythrocyte massAnswer
- BDepletion of maternal iron stores due to fetal demand
- CImpaired absorption of dietary folate in the small intestine
- DSuppression of bone marrow erythrocyte production
- EDestruction of circulating erythrocytes in the reticuloendothelial system
Answer
The hematologic findings are caused by a disproportionate increase in plasma volume relative to erythrocyte mass, known as physiologic anemia of pregnancy.
During normal pregnancy, maternal blood volume undergoes significant changes. Plasma volume increases continuously starting in the first trimester, expanding by approximately 40% to 50% above baseline. Simultaneously, the total red blood cell (erythrocyte) mass also increases, but only by about 20% to 30%. Because the expansion of plasma volume outpaces the production of new red blood cells, hemodilution occurs. This results in a physiologic, normocytic anemia that typically reaches its nadir during the late second or early third trimester. According to the American College of Obstetricians and Gynecologists (ACOG), true anemia in the second trimester is defined as a hemoglobin level strictly less than 10.5 g/dL. Because this patient's hemoglobin is 10.7 g/dL and her mean corpuscular volume (MCV) is perfectly normal (89 fL), her presentation is a classic example of this normal, healthy physiologic adaptation rather than a disease state.
Step-by-Step Solution
Key Concept
Physiologic Anemia of Pregnancy