Primary Cause of Hemodilution in Pregnancy

Physiologic Mechanism

The American College of Hepatology notes that plasma volume expansion occurs disproportionately to red blood cell mass expansion during pregnancy, creating a dilutional effect on hemoglobin and hematocrit values, with blood volume increasing gradually throughout pregnancy 1
The Centers for Disease Control and Prevention recommends considering that hemoglobin concentration and hematocrit decline during the first and second trimesters specifically because of this expanding blood volume 2
The American College of Hepatology states that this hemodilution creates a hyperdynamic circulatory state similar to what is seen in other high-output conditions 1
The American College of Hepatology also notes that albumin levels decrease during the second half of pregnancy due to this hemodilution effect 1

The U.S. Preventive Services Task Force notes that hemodilution and physiologic anemia normally occurs during pregnancy, making hemoglobin or hematocrit measurement alone imprecise for determining true iron deficiency status, with a strength of evidence considered moderate 4, 5
The Centers for Disease Control and Prevention warns that excessively high hemoglobin or hematocrit values (>15.0 g/dL or >45%) in the second trimester likely indicate poor blood volume expansion and are associated with adverse outcomes including preterm delivery and fetal growth retardation 2