Hemodilution During Pregnancy

Physiological Mechanism of Hemodilution

During normal pregnancy, plasma volume increases by approximately 40-50% and red cell mass increases by only 20-30%, creating a physiologic hemodilution, according to the American Journal of Hematology 2
The plasma volume expansion begins after the first trimester and reaches its maximum at approximately 34-36 weeks of gestation, resulting in a decline in hemoglobin concentration and hematocrit during the first and second trimesters, as reported by MMWR Recommendations and Reports 1
Mean platelet counts decrease by approximately 17% during pregnancy due to the dilutional effect, as noted by the American Journal of Hematology 3, 2

Inadequate plasma volume expansion is associated with poor pregnancy outcomes, including hypertension, fetal growth retardation, preterm delivery, and increased risk of stillbirth, as reported by MMWR Recommendations and Reports 1 and Circulation 5
Women with a hematocrit ≥43% at 26-30 weeks' gestation have more than a twofold increased risk for preterm delivery and a fourfold increased risk for fetal growth retardation compared to women with Hct of 33-36%, according to MMWR Recommendations and Reports 1
Elevated hemoglobin concentration or hematocrit (>3 standard deviations above the mean) in the second trimester likely indicates poor blood volume expansion and is associated with adverse outcomes, as noted by MMWR Recommendations and Reports 1 and Circulation 5

Misinterpreting the physiologic hemodilution of pregnancy as pathologic anemia requiring aggressive treatment can lead to over-correction and worse outcomes, as reported by MMWR Recommendations and Reports 1