Primary Cause of Hemodilution in Pregnancy
The primary cause of hemodilution (physiologic anemia) in a healthy pregnancy is increased plasma volume (Answer B). 1
Physiologic Mechanism
Plasma volume expansion occurs disproportionately to red blood cell mass expansion during pregnancy, creating a dilutional effect on hemoglobin and hematocrit values. 1, 2 This is a normal adaptive response that begins early in pregnancy and reaches maximum effect by the third trimester. 1
Key Physiologic Changes:
- Blood volume increases gradually throughout pregnancy, with plasma volume expanding more rapidly than red blood cell mass 1, 2
- Hemoglobin concentration and hematocrit decline during the first and second trimesters specifically because of this expanding blood volume 1
- This hemodilution creates a hyperdynamic circulatory state similar to what is seen in other high-output conditions 1
- Albumin levels decrease during the second half of pregnancy due to this hemodilution effect 1
Why Other Options Are Incorrect:
Decreased red blood cell production (Option A) is not the mechanism - red blood cell mass actually increases during pregnancy, just not as much as plasma volume increases. 1, 2
Increased fetal demands for iron (Option C) is not the primary cause of hemodilution - while iron demands do increase threefold in the second and third trimesters, this contributes to iron deficiency anemia when stores are inadequate, not to the physiologic hemodilution itself. 1
Reduced albumin synthesis (Option D) is a consequence, not a cause - albumin levels decrease during pregnancy due to hemodilution, not because of reduced synthesis. 1
Clinical Significance:
This physiologic hemodilution serves an important adaptive function by improving placental microcirculation and preparing for expected blood loss at delivery. 2, 3 The U.S. Preventive Services Task Force specifically notes that "hemodilution and physiologic anemia normally occurs during pregnancy," making hemoglobin or hematocrit measurement alone imprecise for determining true iron deficiency status. 1
Excessively high hemoglobin or hematocrit values (>15.0 g/dL or >45%) in the second trimester likely indicate poor blood volume expansion and are associated with adverse outcomes including preterm delivery and fetal growth retardation. 1