Preeclampsia is one of the most common complications of pregnancy and is characterized by high blood pressure and protein in the urine. This can cause problems in the second half of pregnancy for both the mother and fetus. This study of preeclampsia consists of two parts: 1) a randomized, placebo controlled, multicenter clinical trial of 10,000 low-risk nulliparous women between 9 and 16 weeks gestation and 2) an observational, cohort study of 4,000 patients between 9 and 12 weeks gestation who are also enrolled in the trial.