Plasma magnesium concentration in the third trimester of pregnancy and its relationship to plasma volume

Summary

Serial determinations of plasma magnesium were made in 30 normal pregnant women during the last 12 weeks of pregnancy. Mean weekly concentrations were relatively constant at about 0·72 mmol/L (s.d.±0.01 mmol/L). Individual values ranged from 0·55 to 0·96 mmol/L (mean±s.d.: 0·72±0·07 mmol/L) and were significantly lower (P < 0·001) than those of 36 non-pregnant control subjects who had a mean concentration of 0·83±0·06 mmol/L. Magnesium concentrations in pregnancy affected by toxaemia (14 patients) were no different from concentrations in normal pregnancy. No correlation was found between magnesium concentration and plasma volume in 31 pregnant patients; in particular, patients (n = 7) with marked plasma hypovolaemia had concentrations no different from those of patients (n = 13) with normal plasma volumes or from the single patient with marked hyper-volaemia. This finding suggests a more complex mechanism is involved in the lowering of plasma magnesium concentration in pregnancy than mere dilution from the gestational increase in plasma volume, as has previously been suggested.