Objective: The purpose of this trial was to determine whether calcium supplementation of pregnant women with low calcium intake reduces preeclampsia and preterm delivery. Study design: Randomized placebo- controlled, double- blinded trial in nulliparous normotensive women from populations with dietary calcium < 600 mg/d. Women who were recruited before gestational week 20 received supplements (1.5 g calcium/d or placebo) through-out pregnancy. Primary outcomes were preeclampsia and preterm delivery; secondary outcomes focused on severe morbidity and maternal and neonatal mortality rates. Results: The groups comprised 8325 women who were assigned randomly. Both groups had similar gestational ages, demographic characteristics, and blood pressure levels at entry. Compliance were both 85% and follow- up losses (calcium, 3.4% ; placebo, 3.7% ). Calcium supplementation was associated with a non- statistically significant small reduction in preeclampsia (4.1% vs 4.5% ) that was evident by 35 weeks of gestation (1.2% vs 2.8% ; P = .04). Eclampsia (risk ratio, 0.68: 95% CI, 0.48- 0.97) and severe gestational hypertension (risk ratio, 0.71; 95% CI, 0.61- 0.82) were significantly lower in the calcium group. Overall, there was a reduction in the severe preeclamptic complications index (risk ratio, 0.76; 95% CI, 0.66- 0.89; life- table analysis, log rank test; P = .04). The severe maternal morbidity and mortality index was also reduced in the supplementation group (risk ratio, 0.80; 95% CI, 0.70- 0.91). Preterm delivery (the neonatal primary outcome) and early preterm delivery tended to be reduced among women who were ≤ 20 years of age (risk ratio, 0.82; 95% CI, 0.67- 1.01; risk ratio, 0.64; 95% CI, 0.42- 0.98, respectively). The neonatal mortality rate was lower (risk ratio, 0.70; 95% CI, 0.56- 0.88) in the calcium group. Conclusion: A 1.5 g calcium/day supplement did not prevent preeclampsia but did reduce its severity, maternal morbidity, and neonatal mortality, albeit these were secondary outcomes. Objective: The purpose of this trial was to determine whether calcium supplementation of pregnant women with low calcium intake reduces preeclampsia and preterm delivery. Study design: Randomized placebo-controlled, double-blinded trial in nulliparous normotensive women from populations with dietary calcium <600 mg / d. Women who were recruited before gestational week 20 received supplements (1.5 g calcium / d or placebo) through-out pregnancy. Primary outcomes were preeclampsia and preterm delivery; Both groups were similar gestational ages, demographic characteristics, and blood pressure levels at entry. Compliance were both 85% and follow-up losses (calcium, 3.4%; placebo, 3.7%). Calcium supplementation was associated with a non-reduced significant small reduction in preeclampsia (4.1% vs 4.5%) that was evident by 35 weeks of gestation (1.2% vs 2.8%; P = .04). Eclampsia (risk ratio, 0.68: 95% CI, 0.48-0.97) and severe gestational hypertension Overall, there was a reduction in the severe preeclamptic complications index (risk ratio, 0.76; 95% CI, 0.66- 0.89; life-table analysis, log rank test; P = .04). The severe maternal morbidity and mortality index was also reduced in the supplementation group (risk ratio, 0.80; 95% CI, 0.70- 0.91). Preterm delivery (the neonatal primary outcome) and early preterm delivery tended to be reduced among women who were ≤ 20 years The neonatal mortality rate was lower (risk ratio, 0.70; 95% CI, 0.56- 0.98, respectively) 0.88) in the calcium group. Conclusion: A 1.5 g calcium / day supplement did not prevent preeclampsia but did reduce its severity, maternal morbidity, and neonatal mortality, albeit thes ewere secondary outcomes.