What characterizes the acid‑base balance in a normal pregnancy?

Acid-Base Balance in Normal Pregnancy

Normal pregnancy is characterized by a fully compensated chronic respiratory alkalosis, driven by progesterone-induced hyperventilation, with arterial PaCO₂ typically 28-32 mm Hg, bicarbonate 18-21 mEq/L, and pH maintained near-normal at 7.40-7.45. 1

Mechanism of Respiratory Alkalosis

• Progesterone directly stimulates central respiratory centers, increasing minute ventilation by approximately 20-40% above baseline by term, independent of chemoreceptor stimulation 1, 2
• This progesterone-mediated increase in minute ventilation exceeds the elevated oxygen demands of pregnancy, producing net carbon dioxide elimination and mild respiratory alkalosis 1
• Maternal oxygen consumption rises by 20-33% in the third trimester to meet metabolic needs of the fetus, placenta, and maternal organs, but the ventilatory response is disproportionately greater 1, 3

Renal Compensation and Metabolic Adaptation

• The kidneys compensate by excreting bicarbonate, resulting in a fully compensated state that is physiologically normal in pregnancy 1
• The compensatory reduction in bicarbonate is primarily sustained by an increase in plasma chloride concentration relative to sodium, creating a relative hyperchloremic state 4
• The strong ion difference (SID) decreases in arterial blood (31.5 vs 36.1 mM in non-pregnant women), mainly due to increased chloride 4

Expected Arterial Blood Gas Values

In normal pregnancy, arterial blood gases show:

• PaCO₂: 28-32 mm Hg (compared to 35-45 mm Hg in non-pregnant adults) 1
• Bicarbonate: 18-21 mEq/L (compared to 22-26 mEq/L in non-pregnant adults) 1
• pH: 7.40-7.45 (slightly alkalotic but within normal range) 1, 5
• Base excess typically -2.8 to -3.3 mEq/L 6

Cerebrospinal Fluid Changes

• The CSF-to-plasma PCO₂ difference is significantly higher in pregnant women (1.5 vs 1.0 kPa in non-pregnant), mainly due to decreased arterial PCO₂ 4
• The CSF-to-plasma difference in strong ion difference is less negative in pregnancy (-7.8 vs -11.4 mM), reflecting chronic acid-base adaptation 4
• The compensatory changes are more pronounced in plasma than in CSF, as the decrease in PCO₂ is more marked in the arterial compartment 4

Clinical Implications

• Respiratory rate remains essentially unchanged during normal pregnancy; a rate >20 breaths per minute should prompt investigation for pathology rather than being attributed to normal physiological changes 1
• This chronic respiratory alkalosis is present from early pregnancy and persists throughout gestation, representing a normal adaptive response 5, 7
• The mild alkalosis should not be corrected, as it represents physiological adaptation; however, excessive maternal alkalosis beyond these normal parameters may potentially compromise fetal oxygenation in borderline situations 8

Important Caveats

• While the overall base excess in pregnancy appears similar to non-pregnant states, there are greater offsetting contributions including hyperchloremic acidosis and hypoalbuminemic alkalosis 6
• The magnitude of these opposing contributors (rather than absolute base excess) may be more clinically relevant when assessing acid-base disturbances in pregnancy 6
• Any deviation from this expected pattern (e.g., metabolic acidosis, severe alkalosis, or normal PaCO₂ values) warrants investigation for pathology 5