Red Blood Cell Count (RBC)
Trimester-Specific Reference Ranges
RBC count typically decreases during pregnancy due to hemodilution. Plasma volume expands by 40–50% whereas red cell mass increases by only 20–30%, producing the **physiologic anemia of pregnancy**.
| Units | Nonpregnant Female | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| x10⁶/µL (x10⁶/mm³) | 4.0–5.2 | 3.42–4.55 | 2.81–4.49 | 2.72–4.43 |
| x10¹²/L | 4.0–5.2 | 3.42–4.55 | 2.81–4.49 | 2.72–4.43 |
Physiologic changes in pregnancy
- Plasma volume expansion exceeds RBC mass expansion → dilutional anemia.
- Greatest hemodilution occurs in the 2nd trimester.
- Iron requirements rise sharply; deficiency is common.
- Markedly low RBC count warrants workup for anemia—not attributed solely to pregnancy.
Causes of low RBC (anemia)
- Physiologic hemodilution of pregnancy
- Iron deficiency anemia (most common)
- Folate or vitamin B12 deficiency
- Thalassemia trait or hemoglobinopathy
- Chronic disease anemia
- Bone marrow suppression
- Hemorrhage (acute or chronic)
- Hemolytic anemia
- Renal disease (↓ erythropoietin)
- Hypothyroidism
Causes of high RBC (erythrocytosis)
- Dehydration or hemoconcentration
- Chronic hypoxia (cyanotic heart disease)
- Smoking or CO exposure
- High altitude residence
- Polycythemia vera (rare)
- Excess erythropoietin production (renal tumor, liver tumor)
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31. PMID: 19935037
- Cunningham FG et al. Williams Obstetrics, 26th ed. Hematologic physiology.
- ACOG Practice Bulletin: Anemia in Pregnancy.