Bicarbonate (HCO₃⁻) — Trimester-Specific Reference Ranges
Serum bicarbonate reflects metabolic acid–base status. Pregnancy lowers bicarbonate slightly due to chronic respiratory alkalosis and renal compensation.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mEq/L (mmol/L) | 22–30 | 18–26 | 18–26 | 18–26 |
Physiologic changes in pregnancy
- Progesterone stimulates ventilation → mild chronic respiratory alkalosis.
- Kidneys excrete bicarbonate to compensate → serum HCO₃⁻ decreases ~3–5 mEq/L.
- Typical bicarbonate in pregnancy: 18–22 mEq/L.
- Helps maintain slightly alkalemic maternal pH (~7.44).
Causes of low bicarbonate (metabolic acidosis)
- Ketoacidosis (diabetic, starvation, alcoholic)
- Lactic acidosis: sepsis, shock, hypoxia
- Renal failure or renal tubular acidosis
- Diarrhea or GI bicarbonate loss
- Toxins: methanol, ethylene glycol, salicylates
- Severe dehydration or shock
Causes of high bicarbonate (metabolic alkalosis)
- Vomiting or nasogastric suction (loss of gastric acid)
- Loop or thiazide diuretics
- Mineralocorticoid excess (hyperaldosteronism, Cushing syndrome)
- Contraction alkalosis from volume depletion
- Excess bicarbonate supplementation
When to repeat or further evaluate
- Unexpectedly low bicarbonate in late pregnancy
- Suspected metabolic acidosis → obtain ABG/VBG, pH, pCO₂, anion gap
- Persistent vomiting or dehydration
- Renal dysfunction or abnormal electrolytes
Reference
Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114(6):1326–1331.