At what stage of pregnancy does dilutional anemia typically occur?

Timing of Dilutional Anemia in Pregnancy

Dilutional anemia in pregnancy begins during the first trimester, progresses throughout the second trimester, and reaches its nadir (lowest hemoglobin point) at approximately 24-28 weeks gestation in the late second trimester, with hemoglobin levels gradually rising toward pre-pregnancy values in the third trimester if adequate iron supplementation is provided. 1, 2

Physiologic Timeline

First Trimester (Weeks 0-13)

• Plasma volume expansion begins with minimal increase initially 3
• Hemoglobin concentration starts to decrease due to early hemodilution 1, 4
• Some evidence suggests a preliminary fall in red cell mass may occur during this period 3
• Anemia is defined as hemoglobin <11.0 g/dL during this trimester 2, 5

Second Trimester (Weeks 14-27)

• Progressive and dramatic plasma volume expansion occurs, increasing by approximately 1250 mL (nearly 50% above non-pregnant levels) 3
• Hemoglobin concentration continues to decline, reaching its lowest point 1, 4
• The nadir typically occurs around 24-28 weeks gestation 6
• Anemia is defined as hemoglobin <10.5 g/dL during this trimester 1, 2, 5
• Red cell mass increases by only 250 mL without iron supplementation (versus 400-450 mL with supplementation), creating the dilutional effect 3

Third Trimester (Weeks 28-40)

• Maximum plasma volume is reached at approximately 34-36 weeks, after which little further increase occurs 3
• In women taking iron supplements, hemoglobin gradually increases toward pre-pregnancy levels 1, 4
• In women not taking iron supplements, hemoglobin remains low throughout this trimester 1
• Anemia is defined as hemoglobin <11.0 g/dL during this trimester 1, 2, 5

Mechanism of Dilutional Anemia

The physiologic anemia of pregnancy results from disproportionate expansion of plasma volume compared to red cell mass 7, 3:

• Plasma volume: Increases by ~1250 mL (48% increase) 3
• Red cell mass: Increases by only 250 mL without iron (18% increase) or 400-450 mL with iron supplementation 3
• Result: Venous hematocrit drops from approximately 40% to 33% in the last trimester 3

This differential change serves a biological purpose: red cell mass rises proportionately to oxygen-carrying needs, while the greater plasma volume increment accommodates increased blood flow to organs requiring less oxygen (skin and kidneys) 3.

Clinical Implications

Screening Timing

• Screen at first prenatal visit to establish baseline 6, 5
• Repeat screening at 24-28 weeks gestation when dilutional anemia is maximal 6, 5

Prevention Strategy

• Initiate 30 mg/day elemental iron supplementation at first prenatal visit to prevent progression beyond physiologic dilution 6, 4
• This early supplementation allows hemoglobin to rise in the third trimester rather than remain suppressed 1

Important Caveat

The frequently observed fall in plasma volume during the last 6 weeks of pregnancy is likely a measurement artifact due to poor tracer mixing when women lie supine and obstruct circulation to lower limbs, not a true physiologic change 3.