Physiologic Anemia After 32 Weeks of Pregnancy
Yes, physiologic (dilutional) anemia can and does persist after 32 weeks of pregnancy, though hemoglobin levels typically begin to rise gradually toward pre-pregnancy values in the third trimester with adequate iron intake. 1
Understanding Physiologic Changes Throughout Pregnancy
The hemodynamic changes of pregnancy create a predictable pattern of hemodilution:
- Blood volume expansion peaks at approximately 32 weeks gestation, with cardiac output reaching 40-50% above baseline by this time 2
- During the first and second trimesters, hemoglobin and hematocrit decrease due to plasma volume expansion outpacing red cell mass 1
- In the third trimester, with adequate iron stores, hemoglobin and hematocrit gradually increase toward pre-pregnancy levels, but this does not mean physiologic anemia completely resolves 1
Defining Anemia at Different Gestational Ages
The diagnostic thresholds for anemia vary by trimester, acknowledging that some degree of hemodilution is physiologic:
- Second and third trimester: Hemoglobin <10.5 g/dL defines anemia 1, 3
- Alternative criteria: Some sources use <11.0 g/dL in the first trimester and <10.0 g/dL in the second and third trimesters 4
This means a hemoglobin of 10.0-10.5 g/dL after 32 weeks represents mild physiologic anemia that is expected and normal 1
The Protective Role of Mild Hemodilution
Moderate hemodilution serves critical adaptive functions in late pregnancy:
- Prevents severe disseminated intravascular coagulation (DIC) during labor and delivery, when physiologic hypercoagulability is at its peak 5
- Research evidence suggests mild anemia may be protective against pregnancy loss, with one study showing women with mild anemia had 90.3% odds of completing pregnancy to live birth compared to non-anemic women 5
- No significant association exists between mild anemia and preeclampsia, placental insufficiency, low birth weight, or premature birth 5
Clinical Management After 32 Weeks
The approach depends on hemoglobin levels and symptoms:
- Hemoglobin ≥10.0 g/dL: Continue routine iron supplementation at 30 mg/day started at first prenatal visit 2, 1
- Hemoglobin <10.0 g/dL but ≥7.0 g/dL: Initiate therapeutic oral iron at 60-120 mg/day 2, 1
- Hemoglobin <7.0 g/dL: Refer to physician for further evaluation and consider transfusion 2
Important Caveats
Do not confuse physiologic anemia with pathologic anemia requiring intervention:
- Hemoglobin >15.0 g/dL or hematocrit >45.0% in the second or third trimester suggests poor blood volume expansion and warrants evaluation for pregnancy complications 2
- If anemia fails to respond to iron therapy after 4 weeks (no increase of 1 g/dL in hemoglobin), further workup with MCV, RDW, and serum ferritin is indicated 2
- Women of African, Mediterranean, or Southeast Asian ancestry with mild anemia unresponsive to iron may have thalassemia minor or sickle cell trait 2
The key distinction is that physiologic anemia after 32 weeks is expected and normal, but hemoglobin should not continue to decline—it should stabilize or gradually improve with iron supplementation. 1