Hematocrit (Whole Blood)
Hematocrit normally decreases during pregnancy due to plasma volume expansion exceeding red cell mass increase (“physiologic dilutional anemia”). Interpretation must consider gestational age, iron status, and underlying hematologic disease.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| % | 35 – 44 | 31 – 41 | 30 – 39 | 28 – 40 |
| Proportion of 1.0 | 0.35 – 0.44 | 0.31 – 0.41 | 0.30 – 0.39 | 0.28 – 0.40 |
Causes of low hematocrit in pregnancy
- Iron deficiency anemia (most common)
- Physiologic hemodilution of pregnancy
- Folate or vitamin B12 deficiency
- Acute or chronic blood loss
- Hemoglobinopathies (sickle cell disease, thalassemias)
- Chronic kidney disease (reduced erythropoietin)
- Bone marrow disorders (aplastic anemia, malignancy)
- Hemolysis (preeclampsia/HELLP, autoimmune hemolysis)
Causes of high hematocrit in pregnancy
- Dehydration and hemoconcentration (most common)
- Smoking or chronic hypoxia
- Congenital heart or lung disease
- Polycythemia vera
- High altitude exposure
- Erythropoietin-producing tumors (rare)
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Obstet Gynecol. 2009.
- ACOG Practice Bulletin No. 233. Anemia in Pregnancy. 2021.
- Milman N. Ann Hematol. 2008.
- Pavord S et al. Br J Haematol. 2020.
- WHO. Haemoglobin concentrations for the diagnosis of anaemia. 2011.
- Peña-Rosas JP et al. Cochrane Database Syst Rev. 2015.