Hypertensive disorders of pregnancy, which include preeclampsia, complicate 2 to 8 percent of pregnancies and are estimated to cause 45,000 maternal deaths annually worldwide. These disorders are also associated with an increased risk of preterm birth, the leading cause of death among children worldwide. 

Calcium supplementation of 1500 to 2000 mg per day, divided into three doses, during pregnancy has been recommended by the World Health Organization (WHO) since 2011 to reduce the risk of preeclampsia in populations with low dietary calcium intake. Previous trials of high-dose calcium supplementation of at least 1000 mg per day reduced the risk of preeclampsia by more than half and the risk of preterm birth by 24 percent. However, only a few countries have implemented routine high-dose calcium supplementation in pregnancy due to adherence concerns and high cost. 

The findings of two independent, randomized trials conducted in India and Tanzania to compare the efficacy of low-dose calcium supplementation (500 mg per day) to high-dose calcium supplementation (1500 mg per day) in reducing the risk of preeclampsia and preterm birth was recently published in The New England Journal of Medicine. In each trial, 11,000 first-time pregnant women were enrolled. The results indicate that low-dose calcium supplementation was as effective as the high-dose in terms of the risk of preeclampsia. The trial in India, but not the one in Tanzania, showed that the low-dose calcium supplement was as effective as the high-dose with respect to preterm birth.