• Magnesium sulphate does not reverse or prevent pre-eclampsia, nor does it significantly lower blood pressure and it is not recommended as an antihypertensive agent (2).
• Monitoring of MgSO4 should utilise clinical parameters of urinary output, respiratory rate, oxygen saturation and patellar reflexes. Serum magnesium levels shoud be measured if toxicity is suspected. This is often apparent with serum levels of >3.5 mmol/l. Toxicity is particularly likely in the presence of significant renal insufficiency. The drug treatment of MgSO4 toxicity is 10% calcium gluconate (1 g) over 10 min (2).
  
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References:

1. Blaubuch Frauenklinik KSA: link (Nr. 59/2)
2. Management of pre-eclampsia. Anaesthesia 2012,67,1009-1020: full text | pdf