Erythrocyte hydration in normal human pregnancy

Objective— To determine whether the fall in plasma osmolality in normal human pregnancy resulted in cellular overhydration. Design— The changes in erythrocyte hydration, potassium and total osmoles in response to a decrease in osmolality in vitro and associated with the fall in plasma osmolality in normal pregnancy were determined. Subjects— Fifty‐one women were studied serially during pregnancy and again 20 weeks after delivery. Results— Erythrocytes from pregnant women exposed in vitro to a 29.9% osmolality decrement had a 28.5% increase in cell hydration. At 14 weeks gestation although plasma osmolality was lower than after delivery (281.1 vs 291.6 mosmol/kg; P < 0.01 ) both erythrocyte hydration (1.83 l/kg dry weight cells) and potassium (264 mmol/kg) contents were reduced from the nonpregnant values (1.88 l/kg; P < 0.001 ). For the remainder of pregnancy plasma osmolality remained at this lower level but cell hydration and potassium both increased to values at 38 weeks gestation that were greater than in the nonpregnant state (1.92 vs 1.88 1/kg; 287 vs 272 mmol/kg). Conclusions— These findings suggest that a loss of cell osmoles may be a primary event affecting cell hydration in pregnancy and plasma osmolality is then reduced to maintain normal cell hydration. Subsequent changes in cell hydration were led by changes in intracellular osmole content.