The investigators propose a preventive strategy that may reduce the risk of placental vascular disease and its negative consequences for both the fetus (e.g., poor fetal growth or stillbirth)and mother (e.g., the hypertensive disorders of pregnancy), and which, in turn, should reduce the need for indicated preterm delivery. This strategy is a multicenter, randomized double-blind, placebo-controlled clinical trial (RCT) comparing magnesium citrate supplementation with placebo, each starting at 12 to 20 weeks gestation and continued until delivery. Magnesium citrate is a safe and inexpensive compound that is easily absorbed by the intestinal tract. The results of this RCT may be especially relevant in low and middle income countries that have high rates of prematurity, and limited resources for acute newborn and maternal care.