Why Pregnant Women Are Prone to Anemia
The correct answer is (d) plasma volume expansion - pregnant women develop physiologic anemia because plasma volume increases disproportionately more than red blood cell mass during pregnancy, resulting in hemodilution. 1
Physiologic Mechanism of Pregnancy-Related Anemia
Plasma volume expansion is the primary physiologic cause of anemia in pregnancy. During normal pregnancy, plasma volume increases by approximately 1250 mL (nearly 50% above non-pregnant levels), while red cell mass increases by only 250-450 mL (18-25% increase). 2 This differential expansion creates a dilutional effect where:
- Hemoglobin concentration and hematocrit decline during the first and second trimesters specifically because of expanding blood volume 1
- The venous hematocrit drops from a non-pregnant average of approximately 40% to about 33% during the last trimester 2
- Maximum plasma volume expansion occurs at 34-36 weeks of gestation 2
Why This Physiologic Change Occurs
The disproportionate plasma volume increase serves important biological functions 2:
- Red cell mass rises proportionately to oxygen-carrying needs (modest increase) 2
- Plasma volume increases much more dramatically to accommodate massive increases in blood flow to organs requiring enhanced perfusion but not necessarily more oxygen (particularly skin and kidneys) 2
- This prepares the maternal circulation for expected blood loss at delivery 3
Iron Deficiency as a Compounding Factor
While plasma volume expansion causes physiologic anemia, iron deficiency is the most common pathologic cause of anemia in pregnancy, accounting for 75% of all pregnancy anemias 3:
- The estimated prevalence of iron deficiency in pregnant women is 18.6%, with 16.2% also having anemia 1
- Among pregnant women who do not take iron supplements, hemoglobin and hematocrit remain low throughout the third trimester 1
- Women with adequate iron intake see gradual rises in hemoglobin during the third trimester toward pre-pregnancy levels 1
Why Other Answer Choices Are Incorrect
- (a) Lacking of minerals: Too vague and non-specific; while iron deficiency contributes, it is not the primary physiologic mechanism 4
- (b) Increasing circulation of progesterone: No evidence supports progesterone as a direct cause of anemia in pregnancy 1
- (c) Acute blood loss: This is not a normal feature of pregnancy until delivery 3
- (e) Increasing estrogen level: Not identified as a mechanism for pregnancy-related anemia 1
Clinical Implications for This Patient
For this 32-week pregnant patient with hemoglobin of 9.5 g/dL: