The hyperventilation that occurs during pregnancy is probably due in part to progesterone stimulating the respiratory center. Lung volume changes and altered compliance may also contribute. The effect is a chronic respiratory alkalosis which is compensated by renal excretion of bicarbonate. Typical blood gases results in the third trimester are:

The reduction in bicarbonate results in a slightly reduced ability to buffer a metabolic acid load. The lower pCO2 would shift the oxygen dissociation curve to the left but the minimal change in pH and the increased 2,3 DPG levels during pregnancy mean the ODC is little altered in position.

Nausea and vomiting occur commonly in the first trimester. This may be severe (hyperemesis gravidarum) and intractable vomiting can cause fluid loss and electrolyte disturbances. The acid-base result is typically a metabolic alkalosis. (Note however the actual acid-base effect of vomiting depends on the actual mix of acidic gastric fluid and alkaline intestinal secretions in the vomitus. Alkalosis does not always occur with prolonged vomiting.)

The pregnant woman is prone to develop elevated ketone levels because:

fasting during pregnancy more rapidly results in hypoglycaemia and low insulin levels

insulin resistance develops as pregnancy progresses (probably due to placental hormones)

Fasting ketosis develops in less than 16 hours in late pregnancy as compared to usually > 24 hours in the non-pregnant female. Ketones can cross the placenta and the foetus can adapt to use them as an energy source. Ketones may be important in myelination in the developing central nervous system. This mild ketosis that occurs with fasting does not seem to have any adverse effect on the mother. It is not certain that the ketosis has adverse effects on the foetus but one early study apparently showed some mild impairment on neuropsychological testing of the delivered babies. The current recommendation is that the mother should have frequent small meals.

Ketoacidosis due to maternal diabetes is more serious and definitely has very serious adverse effects on the foetus.

Other
Diuretic use may cause a metabolic alkalosis. This results in a mixed alkalosis because the hyperventilation has already reduced the pCO2.

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