Normal Hemoglobin and Hematocrit Values in Pregnancy
Normal hemoglobin in pregnancy is ≥11.0 g/dL in the first trimester, ≥10.5 g/dL in the second trimester, and ≥11.0 g/dL in the third trimester, with corresponding hematocrit values of ≥33% in the first trimester and similar proportional decreases in subsequent trimesters. 1, 2
Trimester-Specific Normal Values
First Trimester
- Hemoglobin: ≥11.0 g/dL (values below this define anemia) 1, 2
- Hematocrit: ≥33% 3
- Minimal physiologic hemodilution has occurred at this stage, as plasma volume expansion begins after the first trimester 3
Second Trimester
- Hemoglobin: ≥10.5 g/dL 1, 2
- This lower threshold reflects physiologic hemodilution from expanding blood volume 1
- The nadir of hemoglobin occurs during this period due to maximal plasma volume expansion 1
Third Trimester
- Hemoglobin: ≥11.0 g/dL 1, 2
- In women with adequate iron intake, hemoglobin gradually rises during the third trimester toward pre-pregnancy levels 1
- In women not taking iron supplements, hemoglobin remains low throughout this trimester 1
Physiologic Basis for These Values
The decrease in hemoglobin during pregnancy is primarily due to hemodilution, not true anemia. Plasma volume increases by 40-50% while red cell mass increases by only 20-30%, resulting in a dilutional effect 4. This expansion reaches maximum at 34-36 weeks gestation 3.
Research data from 206 healthy Danish women showed the 5th percentile for hemoglobin in iron-supplemented women was 111 g/dL in the first trimester, 103-109 g/dL in the second trimester, and 105-110 g/dL in the third trimester 5. This supports the guideline-recommended cutoffs.
Critical Warning: High Hemoglobin Values
Hemoglobin >15.0 g/dL or hematocrit >45% during pregnancy is concerning and may indicate poor blood volume expansion, not "good health." 1, 3 This paradoxically abnormal finding is associated with:
- More than twofold increased risk for preterm delivery 1
- Fourfold increased risk for fetal growth restriction 1
- Increased risk of hypertension, fetal death, and low birth weight 1
- Women with hematocrit ≥43% at 26-30 weeks have significantly worse outcomes compared to those with hematocrit 33-36% 3
Important Adjustments and Caveats
Population-Specific Variations
- Altitude: Adjust hemoglobin values upward for women living above 3,000 feet 1, 6
- Smoking: Increases hemoglobin by 0.3-0.7 g/dL depending on intensity; adjust anemia thresholds upward accordingly 1, 6
- Race: Black women may have hemoglobin values 0.5-0.9 g/dL lower than white women as normal physiologic variation, not iron deficiency 1
Optimal Range for Pregnancy Outcomes
Research demonstrates that the minimum incidence of low birth weight and preterm labor occurs with hemoglobin concentrations of 95-105 g/L (9.5-10.5 g/dL) when associated with normal mean corpuscular volume >84 fL 7. This range, while technically meeting criteria for "anemia" by strict definition, represents optimal physiologic adaptation to pregnancy.
Screening Recommendations
Screen all pregnant women at the first prenatal visit and again at 24-28 weeks gestation with a complete blood count. 1 The second screening captures the period of maximal hemodilution when anemia is most likely to manifest 3.