Parenteral magnesium load tests in postpartum American women

The magnesium status of 185 moderate income American mothers was assessed in the immediate postpartum period by the intravenous magnesium load test. Pre- and postload collections of urine were each made for approximately 24 hours because pilot studies revealed significant diurnal variation in magnesium excretion. The magnesium load provided 0.4 to 0.5 mEq of magnesium/kg of estimated lean body weight. The mean retention was 51% +/- 2.2 (SEM). Patients retaining more or less than an arbitrary limit of 40% of the magnesium load were compared. No differences in mean age, weight, or parity were found between the groups. The high retention group reported a diet lower in magnesium and had a significantly lower plasma magnesium value. Magnesium retention of over 90% of the load was found in biologically immature multiparas (less than 17 years) and in young mothers of twins. Among the multiparous patients, those with the longest interval since the previous pregnancy had the lowest retention values. Most of the primiparous patients had met the magnesium requirements of a singleton pregnancy and rejected most of the load, but 6 primiparous women whose active labor exceeded 18 hours had a retention of 77.91% of the load. This was significantly higher than the 45.0 +/- 3.52 (SEM) % retention in 70 primiparous mothers with shorter duration of active labor (P less than 0.005). No other symptoms or complications of pregnancy could be correlated with the magnesium load values. Further definition of the magnesium load test is indicated.