Blood Urea Nitrogen (BUN)

​Blood Urea Nitrogen (BUN) testing is a key marker for evaluating kidney function and protein metabolism during pregnancy. BUN levels can fluctuate due to changes in maternal physiology, diet, and kidney health, making monitoring essential for maternal and fetal well-being. In this article, we’ll examine optimal BUN ranges for pregnancy, explore causes of elevated or decreased levels, and discuss how pregnancy-specific changes influence these values. 

BUN Testing

Blood urea nitrogen (BUN) is a blood test that measures the amount of nitrogen in the form of urea in the bloodstream. 

Urea is a waste product produced in the liver when the body breaks down proteins. The kidneys excrete it through urine. BUN levels are used as a marker of kidney function. The kidneys filter BUN and other waste products through a complex process involving specialized structures and filtration mechanisms.

The initial filtration of BUN and other waste products occurs in tiny, tuft-like clusters of capillaries known as the glomeruli. The blood is under high pressure inside the glomerulus due to the narrowing of the efferent arteriole (the blood vessel that exits the glomerulus). A three-layer barrier selectively allows small molecules such as water and waste products like BUN to pass through while restricting the passage of larger molecules like proteins. BUN is a small waste product derived from the breakdown of proteins, so it easily passes through this filtration barrier.

As the blood is filtered in the glomerulus, a filtrate of water, electrolytes, and waste is formed in Bowman’s capsule, which surrounds each glomerulus. This filtrate continues to flow through the renal tubules, concentrating more as components are reabsorbed. The final filtrate is then excreted as urine. 

BUN in Pregnancy

Increased glomerular filtration during pregnancy results in more efficient filtering of waste products, including BUN, from the bloodstream. Clearance increases in the first trimester and continues throughout pregnancy, only increasing again postpartum.

As a result, BUN levels may decrease or remain within the lower end of the normal reference range.

After childbirth, there is a gradual normalization of blood volume. This can lead to a concentration of blood constituents, including BUN, and result in an increase in BUN levels postpartum. The increase in BUN levels postpartum typically occurs within the first few days to a week after giving birth and normalizes by six weeks postpartum.

High BUN in Pregnancy

In contrast to normal pregnancy, where BUN and creatinine decrease, preeclamptic women have BUN and creatinine levels like nonpregnant women, due to reduced GFR. 

BUN ≥9mg/dL can be a biomarker of preeclampsia, generally with elevated serum creatinine and uric acid levels. 

Most Common Causes of High BUN in Pregnnacy

• Dehydration
• Renal Damage
• Kidney/Urinary Tract Infections
• Certain Medications (corticosteroids, antibiotics, NSAIDs)
• Low Progesterone
• Mycotoxin Exposure
• IBD Flare w/Internal Bleeding (Crohn's, Ulcerative Colitis)

Low BUN in Pregnancy

​​There is limited data on the application of this indice in pregnancy.

Most Common Causes of Low BUN in Pregnancy

• Low Protein Diets
• Hypothyroidism
• Recent IV Fluids
• Nutritional Deficiencies (vitamin B6 (pyridoxine), zinc)

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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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