Randomized, double-blind, multicenter. Healthy nulliparous patients were randomly assigned to receive either 2 grams of supplemental calcium daily ((n = 2,295) or placebo (n = 2,294) in a double-blind study. Study tablets were administered beginning from 13 to 21 completed weeks of gestation and continued until the termination of pregnancy. Eligible patients entered a run-in period of 6 to 14 days to exclude highly noncompliant subjects. During the run-in, obstetrical ultrasound was performed if it had not been obtained previously, and blood was drawn for serum calcium and creatinine. Follow-up visits were scheduled every four weeks through the 29th week of gestation, then every two weeks through the 35th week, and weekly thereafter. Blood pressure and urine-protein were obtained at each clinic visit, during labor and delivery, and during the first 24 hours postpartum. Primary endpoints included pregnancy-associated hypertension, pregnancy-associated proteinuria, pre-eclampsia, eclampsia, or hypertension. Other endpoints included placental abruption, cerebral hemorrhage or thrombosis, elevated liver enzymes, acute renal failure, and disseminated intravascular coagulation. Surveillance was conducted for renal calculi. Recruitment began in May 1992 and ended in March 1995. Follow-up was completed in October of 1995. Data analysis continued through March 2000 under the NICHD contract N01HD13121.