 Patients randomized to the magnesium sulfate arm of the study will receive a 4-6 gram intravenous loading dose per institution protocol over at least 20 minutes, and this will be followed by a continuous infusion of 1-2 grams per hour. Magnesium sulfate levels will be recorded as per the usual protocol of the participating hospital only in those hospitals where this is the protocol. Magnesium sulfate levels are not mandatory as long as there is an active protocol for determining the presence or absence of magnesium sulfate toxicity. Magnesium sulfate therapy will be discontinued 24 hours after delivery. If the blood pressure is not controlled reduction in BP to below 160 mmHg systolic and 110 mmHg diastolic within 40 minutes of beginning magnesium sulfate therapy, the patient will be given an intravenous dose of 5mg hydralazine, which can be repeated every 20 minutes as necessary to control the blood pressure. If the patient still needs further BP control she should be given anything EXCEPT labetalol for BP control, i.e. nifedipine, hydralazine, etc. Persistent blood pressure readings above 160 systolic andor 110 diastolic are a protocol violation. 