Red Cell Size Distribution Width (RDW)

In pregnancy, RDW has the potential to serve as a valuable biomarker for disease, as fluctuations in RBC size may reflect changes in nutritional status or physiological adaptations. Despite its informative nature, RDW is rarely leveraged as a standalone diagnostic tool, highlighting the need for greater awareness and understanding of its implications in maternal health.

RDW Testing

RDW measures the variability in red blood cell size. The lab analysis of the size is not based on the actual width of the cell but the width of the curve of the red blood cell.

RDW-SD and RDW-CV are often analyzed as part of the red blood cell blood analysis:

• RDW-CV = Coefficient of Variation
• RDW-SD = Standard Deviation

We will talk about RDW-CV in this section. 

RDW-CV = (Standard Deviation of MCV / MCV) × 100

This gives us a percentage value. 

RDW in Pregnancy

There is a rise in RDW in the last four to six weeks of pregnancy, with no known etiology. Early increases in RDW are associated with pregnancy loss, so it is theorized that this may be a biological change close to parturition.

The increase in RDW is primarily driven by an increased release of nucleated red blood cells, as well as younger reticulocytes. 

Some variability is normal, as it represents a healthy balance between young reticulocytes and older mature red blood cells in circulation. A release of nucleated red blood cells (nRBC) into circulation can cause larger variation in RDW, as will a decrease in red blood cell production.

High RDW in Pregnancy

Higher RDW levels (>14.5%) have been associated with higher risks of preeclampsia, gestational diabetes, and cholestasis.

A sudden rise in RDW and mean platelet volume (MPV) is a biomarker for preterm labor risk.
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A sudden rise in RDW is a biomarker for intrahepatic cholestasis of pregnancy (ICP).

Most Common Causes of High RDW in Pregnancy

• Nutritional Deficiencies (Iron, Vitamin B12 (Cobalamin)
• Hypoxia
• Sleep Apnea/Snoring
• Asthma
• Thalassemia
• Sickle Cell Anemia
• Hemolysis
• Oxidative Stress/Inflammation
• Liver Disease/Inflammation
• T3 Syndrome/Subclinical Hypothyroidism
• Autoimmune Disease
• Myeloproliferative Diseases (Polycythemia)
• Genetics

Low RDW in Pregnancy

When the RDW is low, this indicates a decrease in the variability of the red blood cell sizes: a lower percentage are not shaped the same.

​There is a normal amount of variation in red blood cell size, as younger blood cells are slightly larger. But when the percentage is too low, this can indicate that there is not enough new RBC production.

Most Common Causes of Low RDW in Pregnancy

• Nutritional Deficiencies (Vitamin B6 (Pyridoxine), copper, zinc)
• Aplastic Anemia
• Inflammation/Chronic Disease
• Genetics

Anything that decreases overall red blood cell production can cause a lower RDW by decreasing the release of young red blood cells. See RBC article. 

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Medical Disclaimer:
This content is for educational purposes only and is intended for licensed medical professionals. It is based on current research, subject to change, and not intended to diagnose, treat, or prevent any medical condition. This information does not replace consultation with a qualified healthcare provider. Seek professional medical evaluation and treatment from a licensed provider. Use of this information is at your own risk.

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