Red Cell Distribution Width (RDW)
Trimester-Specific Reference Ranges
RDW reflects the variability in red blood cell size (anisocytosis). During pregnancy, RDW is usually normal or mildly elevated due to hemodilution, increased iron demand, and physiologic reticulocytosis.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| % | <14.5 | 11.7–14.9 | 12.3–14.7 | 11.4–16.6 |
Physiologic changes in pregnancy
- Hemodilution may slightly increase RDW.
- Iron requirements rise, sometimes preceding supplementation.
- Higher reticulocyte turnover contributes to variability in RBC size.
- Marked RDW elevation indicates anemia or nutritional deficiency.
Causes of high RDW
- Iron deficiency anemia (most common in pregnancy)
- Folate deficiency
- Vitamin B12 deficiency
- Mixed nutritional anemias
- Hemolysis or hemoglobinopathies
- Chronic inflammatory anemia
- Acute or recent blood loss
Causes of low RDW (uncommon)
Low RDW is rare and usually of limited clinical relevance.
- Uniform microcytosis
- Early iron deficiency prior to anisocytosis
References
- Lockitch G. Handbook of Diagnostic Biochemistry and Hematology in Normal Pregnancy. CRC Press; 1993.
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31. PMID: 19935037