Changes in osmolality and blood serum ion concentrations in pregnancy

Changes in osmolality and the concentration of cations (Na, K, Ca, Mg) were studied in blood serum of pregnant women from two weeks after conception, throughout the whole pregnancy, and within the first week after delivery. Altogether 239 women from 18 to 40 years of age were studied. Blood serum osmolality decreased from 287±0.8 to 278±1.6 mOsm/kg H₂O from the fifth week of pregnancy and remained virtually at this level until the end of pregnancy. Hyponatremia was found during the three trimesters of pregnancy, in trimesters II and III hypokalemia was not observed, whereas hypocalcemia and hypomagnemia were found. On the first day after delivery, the blood serum osmolality and concentrations of magnesium ions returned to their levels in nonpregnant women, whereas concentrations of sodium and calcium ions remained decreased. No correlation was found between hypoosmia and changes in blood serum concentrations of ions under study during the three trimesters of pregnancy. Thus, in normal pregnancy, hypoosmia develops from the fifth week after conception and persists until delivery. The concentrations of sodium, potassium, calcium, and magnesium ions are regulated by independent mechanisms to provide retention of these parameters within certain periods of pregnancy at the level of nonpregnant women on the background of hypoosmia.