There were three phases in the study. The first, a retrospective cohort study, examined the incidence rate of pulmonary edema when MgSO4 was administered as a tocolytic agent. The second, a case-control study nested within the cohort, examined the relationship between the patient and other clinical factors and the incidence of pulmonary edema in patients treated with MgSO4. The third phase used the results of the case-control study to develop and validate a predictive index to define a group of patients at increased risk of developing pulmonary edema associated with MgSO4.