Erythrocyte ion and water balance and membrane potential in the puerperium of normal pregnancy.
OBJECTIVE: To examine the balance of erythrocyte ions and water during the rapid changes in plasma osmolality in the early puerperium, and during the subsequent period of sustained readjustment. DESIGN: A serial study from the third trimester of pregnancy to 20 weeks after delivery. PARTICIPANTS: Thirty-five primiparous women who had experienced no antenatal complications. MAIN OUTCOME MEASURES: Plasma osmolality, erythrocyte hydration, potassium, chloride and sodium were measured and nondiffusible ion content and erythrocyte membrane potential calculated. Plasma sodium, potassium and chloride were also measured. RESULTS: During the first week after delivery plasma osmolality increased (280 (SEM 0.52)-289 (SEM 0.64) mosmol/kg; P