Magnesium Sulfate in Obese Preeclamptics

The purpose of this study is to learn how medications participant will receive in the course of their delivery are metabolized (broken down by the body). The investigators hope to learn more about the way drugs are metabolized by pregnant women, and how those drugs are distributed in the blood and body compartments and cleared in the urine, and how maternal body weight affects the metabolism. With this information the investigators will be able to develop a treatment regimen considering factors in the body that affect magnesium levels so that the investigators can improve treatment of pregnant women. The goal of the study is to understand how different dosing of magnesium sulfate affects blood levels of the magnesium in larger women. Magnesium sulfate is generally given to patients to protect the mother against seizures in the case of preeclampsia. Different hospitals in the United States use different doses of magnesium sulfate. Participants will receive one of two accepted doses of magnesium sulfate for preeclampsia. The dose is usually 4g - 6g at the start, with another dose of 2-3 g every hour until 24 hours after delivery. Participants will be assigned to either a dose of 4 g at the start, followed by 1g every hour; or a dose of 6 g at the start, followed by 2g every hour until treatment for preeclampsia is complete. Participants will receive magnesium sulfate in the course of their normal clinical care whether or not they participate in this study.