A pregnant mother undergoes significant changes in acid-base status as well as sodium and calcium metabolism to combat her physiological needs of pregnancy. Pregnant patients experience mild respiratory alkalosis due to the stimulation of the respiratory center by progesterone. This is associated with a corresponding increase in bicarbonate excretion by kidneys; as a result, the pH remains slightly high (7.40–7.45) but within the normal range. Pregnant women are predisposed to starvation ketosis as compared to nonpregnant states due to relative insulin resistance and increased production of the counter-regulatory hormone. Physiological mild hyponatremia occurs during pregnancy due to increased AVP secretion caused by resetting of osmoreceptors in the hypothalamus at a lower osmolality, but values below 130 mEq/L require a diagnostic workup and intervention. Gestational diabetes insipidus can occur due to increased production or decreased destruction of enzyme vasopressinase. Secretion of parathyroid hormone-related peptide by the placenta and breasts and two- to three-fold increased calcium and phosphate absorption in the maternal gut are the key changes in calcium metabolism during pregnancy. Though rare, both hypo- and hypercalcemia in pregnancy are associated with significant maternofetal morbidity and mortality.
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