Why Physiologic Anemia Occurs in Pregnancy
Physiologic anemia in pregnancy occurs because plasma volume expands much more rapidly and extensively than red blood cell mass, resulting in hemodilution. 1
Mechanism of Hemodilution
The fundamental cause is a disproportionate expansion of blood components during pregnancy:
- Plasma volume increases by approximately 1250 mL, while red cell mass increases by only 250-450 mL, according to the CDC 1
- This creates a dilutional effect where the concentration of hemoglobin and hematocrit decreases despite an actual increase in total red blood cells 1
- The hemodilution is most pronounced during the first and second trimesters, when plasma volume expansion outpaces red cell production 2, 1
Physiologic Purpose
This hemodynamic adaptation serves important functions:
- Prepares the maternal circulation for expected blood loss at delivery by increasing total blood volume 3
- The expanded plasma volume supports increased metabolic demands of pregnancy and placental perfusion 3
Distinction from Pathologic Anemia
It is critical to differentiate physiologic hemodilution from true iron deficiency anemia:
- Physiologic anemia results purely from dilution with normal iron stores 2, 1
- Iron deficiency anemia accounts for 75% of all pregnancy anemias and represents a pathologic state requiring treatment 1, 3
- Among pregnant women who do not take iron supplements, hemoglobin and hematocrit remain low throughout the third trimester, suggesting that iron deficiency commonly compounds the physiologic hemodilution 1
Clinical Implications
The physiologic nature of pregnancy-related hemodilution creates diagnostic challenges:
- Using hemoglobin or hematocrit measurement alone to determine iron deficiency status can be imprecise due to the normal hemodilution 2
- Diagnostic thresholds must be adjusted by trimester: first trimester <11.0 g/dL, second trimester <10.5 g/dL, third trimester <11.0 g/dL 4
- Despite physiologic hemodilution being normal, all pregnant women should receive 30 mg/day elemental iron starting at the first prenatal visit to prevent progression to true iron deficiency anemia 4, 5