Physiologic Anemia in Pregnancy

Causes and Mechanisms

• Plasma volume expansion is the primary physiologic cause of anemia in pregnancy, resulting in hemodilution, with plasma volume increasing by approximately 1250 mL and red cell mass increasing by only 250-450 mL, according to the Centers for Disease Control and Prevention (CDC) 1, 2
• Hemoglobin concentration and hematocrit decline during the first and second trimesters specifically because of expanding blood volume, as reported by the CDC 1, 2

Iron Deficiency and Anemia

• Iron deficiency is the most common pathologic cause of anemia in pregnancy, accounting for 75% of all pregnancy anemias, with an estimated prevalence of iron deficiency in pregnant women being 18.6%, as stated by the American Academy of Family Physicians 3, 4, 5
• Among pregnant women who do not take iron supplements, hemoglobin and hematocrit remain low throughout the third trimester, according to the CDC 1, 2

Treatment and Recommendations

• Iron supplementation with 60-120 mg elemental iron daily is appropriate treatment for iron deficiency anemia in pregnancy, as recommended by the US Preventive Services Task Force (USPSTF) and the CDC 3, 4, 6, 7

Physiologic Anemia and Hemodilution in Pregnancy

Mechanism of Hemodilution

• In pregnant individuals, the greatest dilutional effect occurs during the first and second trimesters because plasma volume expands more rapidly than red‑cell mass, leading to lower hemoglobin and hematocrit concentrations despite an overall increase in red‑cell quantity. 8

Definition

• Physiologic anemia of pregnancy is defined as a reduction in hemoglobin/hematocrit that is solely due to hemodilution, while iron stores remain normal. 8

Diagnostic Considerations

• Relying only on hemoglobin or hematocrit values to diagnose iron‑deficiency anemia in pregnancy is unreliable, since normal physiologic hemodilution can mask true iron deficiency. 8

Physiologic (Dilutional) Anemia at Full Term

Mechanism of Dilutional Anemia

• The dilutional effect is most pronounced during the first and second trimesters when plasma volume expands more rapidly than red‑cell production, and it persists through full term. 9

Hemoglobin Thresholds at Full Term

• At 40 weeks gestation, hemoglobin values < 11.9 g/dL meet diagnostic criteria for anemia, distinguishing pathologic anemia from normal physiologic changes. 9
• In the third trimester, hemoglobin < 11.0 g/dL is classified as anemic, whereas values between 11.0–11.9 g/dL at 40 weeks represent the upper boundary of normal physiologic hemodilution. 9

Hemoglobin Dynamics with Iron Supplementation

• Among pregnant individuals who do not receive iron supplementation, hemoglobin remains low throughout the third trimester due to persistent hemodilution; those who receive supplementation show a gradual rise toward pre‑pregnancy levels. 9

Physical Activity Recommendations

• Asymptomatic individuals with physiologic anemia can safely continue moderate‑vigorous physical activity throughout pregnancy, including at full term. 10
• If hemoglobin is ≥ 10.0 g/dL and the individual is asymptomatic, there are no restrictions on exercise intensity. 10
• Even when hemoglobin is < 10.0 g/dL, asymptomatic individuals should maintain physical activity while receiving therapeutic iron supplementation. 10