Hemoglobin Changes Begin in the First Trimester of Pregnancy
Hemoglobin concentration begins to decrease as early as the first trimester of pregnancy, with measurable declines occurring within the first 8-12 weeks of gestation. 1, 2
Timeline of Hemoglobin Decline
The physiologic decrease in hemoglobin starts immediately in early pregnancy and follows a predictable pattern:
- First trimester (weeks 1-12): Hemoglobin begins declining from pre-pregnancy baseline, with the hemodilution process commencing during this period 1
- Nadir point: The lowest hemoglobin concentration occurs in the second trimester, specifically around 24-28 weeks of gestation 1, 2
- Third trimester: Hemoglobin gradually increases toward pre-pregnancy levels in women taking iron supplements, but remains low in those without supplementation 1, 3
Magnitude of the Decline
The total fall in hemoglobin from first to third trimester averages 14 g/dL (approximately 11% of the first trimester value), which is substantially higher than the commonly quoted 5 g/dL decline 4. When excluding outliers and women with medical complications, the population fall is approximately 10.2 g/dL (7.8%) 4.
Physiologic Mechanism
The hemoglobin decrease occurs due to two primary mechanisms:
- Hemodilution: Plasma volume expands by 40-50% while red cell mass increases only 20-30%, creating a dilutional effect that begins in the first trimester and peaks at 34-36 weeks 1, 2
- Iron depletion: Progressive utilization of iron stores throughout pregnancy, particularly in the latter half, contributes to declining hemoglobin beyond simple dilution 3, 5
Clinical Implications for Early Detection
Women with low iron stores (serum ferritin <20 μg/L) before pregnancy show different hemoglobin patterns: their hemoglobin may actually increase slightly in early pregnancy while ferritin remains near zero, indicating absent iron stores to mobilize 5. In contrast, women with adequate iron stores (ferritin ≥20 μg/L) show the expected hemoglobin decline with initially rising then falling ferritin levels 5.
Screening Recommendations
- First prenatal visit: Screen all pregnant women with complete blood count to establish baseline hemoglobin 1, 2, 6
- 24-28 weeks gestation: Repeat screening to capture the period of maximal hemodilution when anemia is most likely to manifest 2, 7, 6
- First trimester hemoglobin predicts third trimester anemia: Hemoglobin levels measured at approximately 12 weeks are the best predictors of anemia in the third trimester (AUC 0.63, sensitivity 65%, specificity 65%) 8
Trimester-Specific Anemia Thresholds
Because hemoglobin naturally declines during pregnancy, different cutoffs define anemia at each stage:
- First trimester: Hemoglobin <11.0 g/dL 1, 2, 6
- Second trimester: Hemoglobin <10.5 g/dL 1, 2, 6
- Third trimester: Hemoglobin <11.0 g/dL 1, 2, 6
Prevention Strategy
Start oral iron supplementation at 30 mg/day at the first prenatal visit for all pregnant women to prevent iron deficiency anemia as pregnancy progresses 2, 7. Women with serum ferritin <50 μg/L in early pregnancy require iron supplements, while those with ferritin >80 μg/L are unlikely to need supplementation 3.
Critical Pitfall to Avoid
High hemoglobin levels (>14.6 g/dL or hematocrit >45%) during pregnancy are concerning, not reassuring. Elevated hemoglobin at first antenatal measurement is associated with increased risk of stillbirth, and failure of normal hemodilution indicates poor blood volume expansion linked to hypertension, fetal growth restriction, and preterm delivery 1, 7.