Urea Nitrogen (blood)
Blood urea nitrogen (BUN) decreases significantly during pregnancy because of increased plasma volume, enhanced renal plasma flow, and increased glomerular filtration rate (GFR). Low BUN in pregnancy is normal.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mg/dL | 7 – 20 | 7 – 12 | 3 – 13 | 3 – 11 |
| mmol/L | 2.5 – 7.1 | 2.5 – 4.3 | 1.1 – 4.6 | 1.1 – 3.9 |
Pregnancy physiology
- BUN decreases due to increased GFR and marked hemodilution.
- Maternal plasma volume expands 40–50%, lowering nitrogenous waste concentrations.
- Low BUN in pregnancy is physiologic unless accompanied by symptoms of liver disease or malnutrition.
Causes of elevated BUN
- Dehydration or volume depletion
- Acute or chronic kidney disease
- Preeclampsia or hypertensive renal involvement
- Upper GI bleeding (↑ protein load)
- High-protein diet or catabolic states
- Heart failure or reduced renal perfusion
Clinical note: Elevated BUN in pregnancy warrants evaluation of creatinine, urine protein, and blood pressure to assess for renal disease or preeclampsia.
Causes of low BUN
- Normal pregnancy physiology
- Liver disease – decreased urea synthesis
- Severe malnutrition or low-protein intake
- Syndrome of inappropriate antidiuretic hormone (SIADH)
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.
- Gilstrap LC & Cunningham FG. Williams Obstetrics, Renal Physiology in Pregnancy.
- Davison JM, Dunlop W. Renal hemodynamics in normal pregnancy. Kidney Int.