Physiologic (Dilutional) Anemia at Full Term
Yes, all pregnant women develop physiologic anemia at full term due to hemodilution from disproportionate plasma volume expansion compared to red cell mass increase. 1, 2, 3
Mechanism of Dilutional Anemia
Physiologic anemia occurs because plasma volume expands by approximately 40-50% (about 1250 mL) while red cell mass increases only 20-30% (250-450 mL), creating relative hemodilution. 2, 3 This dilutional effect is most pronounced during the first and second trimesters when plasma volume expands more rapidly than red cell production, but persists through full term. 1, 3
The key distinction is that physiologic anemia represents normal hemodilution with adequate iron stores, not true iron deficiency. 3
Hemoglobin Thresholds at Full Term
At full term (40 weeks gestation), hemoglobin values below 11.9 g/dL meet diagnostic criteria for anemia. 1 However, this threshold distinguishes pathologic anemia from normal physiologic changes—it does not mean that all women with hemoglobin above this level lack dilutional anemia. 4
In the third trimester, hemoglobin below 11.0 g/dL is considered anemic, while values between 11.0-11.9 g/dL at 40 weeks represent the upper boundary of normal physiologic hemodilution. 1, 4
Clinical Implications
Iron Supplementation Requirements
All pregnant women should receive 30 mg elemental iron daily starting at the first prenatal visit and continuing through full term, regardless of hemoglobin values, because dietary intake cannot meet the three-fold increase in iron requirements during pregnancy. 4 This universal prophylaxis addresses the physiologic demands of pregnancy, not just treatment of anemia. 4
Distinguishing Physiologic from Pathologic Anemia
Relying solely on hemoglobin or hematocrit to diagnose iron deficiency is unreliable because normal physiologic hemodilution can mask true iron deficiency. 3 Among pregnant women who do not take iron supplements, hemoglobin remains low throughout the third trimester due to persistent hemodilution, whereas those taking supplements show gradual increases toward pre-pregnancy levels. 1, 3
Activity Recommendations
Women with physiologic anemia who are asymptomatic can safely continue moderate-vigorous physical activity throughout pregnancy, including at full term. 1 If hemoglobin is above 10.0 g/dL and the woman is asymptomatic, there are no restrictions on exercise intensity. 1, 4 Even with hemoglobin below 10.0 g/dL, if asymptomatic, women should maintain physical activity while receiving therapeutic iron supplementation. 1, 4
Common Pitfalls to Avoid
Do not dismiss all anemia at full term as "just physiologic" without ensuring adequate iron supplementation has been provided throughout pregnancy. 2 While dilutional anemia is universal, superimposed iron deficiency is extremely common, affecting 18.6% of pregnant women and accounting for 75% of pregnancy anemias. 3, 5
Do not assume that hemoglobin values in the "normal" range for full term (11.0-11.9 g/dL) mean the patient is not experiencing symptoms from relative anemia. 2 Symptomatic patients with borderline values may benefit from therapeutic iron dosing (60-120 mg daily) rather than prophylactic dosing (30 mg daily). 2, 4
Hemoglobin above 15.0 g/dL or hematocrit above 45% in the third trimester is abnormal and suggests inadequate plasma volume expansion, which is associated with pregnancy complications. 4 This requires evaluation, not reassurance. 4