Preterm labor is a major public health concern in the United States. The delay of labor, particularly for idiopathic preterm labor, is important because it allows further fetal development and the opportunity to administer steroids which improve fetal lung maturity. It is now common for MgSO4 to be administered to delay labor. The most dangerous adverse event with MgSO4 treatment is the development of pulmonary edema which may lead to maternal adult respiratory distress syndrome and death with associated fetal morbidity and mortality. The estimates of the frequency of this complication vary from 0 to 8 percent. Therefore, improved understanding of the pulmonary toxicity of this therapeutic agent is clinically important.