An expectant mother in her first pregnancy presents to the clinic at 24 weeks of gestation… — USMLE Step 2 CK

An expectant mother in her first pregnancy presents to the clinic at 24 weeks of gestation for a scheduled prenatal evaluation. She reports feeling generally well but notes mild fatigue and bilateral ankle swelling after long shifts at her retail job. She takes a standard daily prenatal vitamin. Vital signs show a blood pressure of $112/68 \text{ mm Hg}$ , pulse of $82/\text{min}$ , and respiratory rate of $16/\text{min}$ . Physical examination reveals trace bilateral pedal edema but is otherwise unremarkable. Laboratory studies return the following results:

Laboratory Parameter	Patient Result
Hemoglobin	$10.7 \text{ g/dL}$
Hematocrit	$32.5\%$
Mean corpuscular volume	$89 \text{ fL}$
Leukocyte count	$11,500/\text{mm}^3$
Platelet count	$220,000/\text{mm}^3$

Which of the following is the most appropriate action regarding these laboratory findings?

Maintain current standard of care with no additional hematologic testingAnswer
B
Initiate therapeutic oral ferrous sulfate supplementation
C
Evaluate total iron-binding capacity and serum ferritin levels
D
Perform hemoglobin electrophoresis screening
E
Request a peripheral blood smear analysis

Answer

Maintain current standard of care with no additional hematologic testing

The correct approach is to maintain the current standard of care. This patient is at 24 weeks of gestation (the second trimester) and has a hemoglobin concentration of

10.7 \text{ g/dL}

. During a normal pregnancy, maternal plasma volume expands by approximately 40% to 50%, while the total red blood cell mass increases by only 20% to 30%. This disproportionate expansion creates a dilutional effect, leading to physiologic anemia. To account for this, the American College of Obstetricians and Gynecologists (ACOG) defines anemia in pregnancy using lower hemoglobin cutoffs:

< 11.0 \text{ g/dL}

in the first and third trimesters, and

< 10.5 \text{ g/dL}

in the second trimester. Because this patient's hemoglobin is greater than

10.5 \text{ g/dL}

, she does not have clinical anemia. Furthermore, her mild leukocytosis (

11,500/\text{mm}^3

) and trace pedal edema are normal adaptations to pregnancy. Therefore, reassurance and continuation of routine prenatal vitamins are sufficient.

Step-by-Step Solution

Identify the patient's current gestational age and relevant laboratory parameters.

The patient is at 24 weeks of gestation (second trimester) with a hemoglobin of 10.7 g/dL.

Gestational age determines the specific physiologic thresholds for expected laboratory changes during pregnancy.

Compare the patient's hemoglobin level to trimester-specific diagnostic thresholds for true anemia.

The lower limit of normal hemoglobin is 11.0 g/dL in the first and third trimesters, and 10.5 g/dL in the second trimester. The patient's hemoglobin (10.7 g/dL) is strictly within normal limits for her trimester.

Applying the correct threshold differentiates between true pathologic anemia (usually iron deficiency) and normal physiologic dilutional anemia.

Determine the appropriate clinical management based on the findings.

Since all laboratory findings (including mild leukocytosis) are normal adaptations to pregnancy, no further intervention is needed.

Unnecessary diagnostic testing and supplementation should be avoided in healthy pregnancies exhibiting normal physiologic adaptations.

Key Concept

Physiologic dilutional anemia in pregnancy and trimester-specific hemoglobin thresholds.

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