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. Rarely, 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 but ketosis may also occur if oral intake is poor. 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 ketosis develops in less than 16 hours in late pregnancy as compared to usually greater than 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 or foetus. There is no information on which to base treatment of ketosis in labouring women.1
Ketoacidosis due to maternal diabetes is more serious and can have very serious adverse effects on the foetus.
Diuretic use may cause a metabolic alkalosis. This results in a mixed alkalosis because the hyperventilation has already reduced the pCO2.