Lower than the WHO-recommended calcium supplementation (equivalent to one 500-milligram pill per day) may help prevent preeclampsia and preterm birth -- common complications in pregnancy that can be fatal to women and newborns, suggests a large study on pregnant women in India and Tanzania.

Calcium supplementation is estimated to halve a pregnant woman's risk of developing preeclampsia and reduce an infant's risk of being born preterm by 25 per cent.

However, the study by Harvard University researchers is the first to evaluate the efficacy of a low-dose regimen versus a high-dose regimen currently recommended by the WHO (equivalent to three 500-milligram pills taken throughout each day).

The study, published in The New England Journal of Medicine, found that low-dose calcium supplementation was as effective as high-dose calcium supplementation in preventing the risk of preeclampsia.

"Overall, our findings show that a single pill per day can be as effective as three," said joint first author Christopher Sudfeld, Associate Professor of global health and nutrition.

"With a reduced pill burden for women and lower costs for governments and programmes that buy calcium pills, calcium supplementation should be considered widely implementable in the places it's needed most -- and should start saving thousands of maternal and newborn lives," Sudfeld added.

The researchers conducted two randomised, double-blind trials of 11,000 pregnant women in India and 11,000 pregnant women in Tanzania to assess if 500mg of calcium per day was as effective as 1,500mg of calcium per day in reducing the risks of preeclampsia and preterm birth (defined as birth before 37 weeks of gestation).

All of the participants were pregnant for the first time, putting them at higher risk for preeclampsia. Starting at less than 20 weeks of pregnancy, they received monthly supplies of daily calcium supplementation, consisting of either three 500mg calcium pills or one 500mg calcium pill and two placebo pills.

Their health was monitored during clinic visits each month of their pregnancy, at delivery, and at six weeks postpartum.

In the India trial, the incidence of preeclampsia was 3.0 per cent among women taking 500mg of calcium daily and 3.6 per cent among women taking 1,500mg of calcium daily. In the Tanzania trial, the incidence of preeclampsia was 3.0 per cent and 2.7 per cent, respectively.

The findings on preterm birth were mixed. In the India trial, the incidence of preterm birth was 11.4 per cent among women taking 500mg of calcium daily and 12.8 per cent among women taking 1,500mg of calcium daily, indicating a similar effect of the two doses. In the Tanzania trial, the incidence of preterm birth was slightly different: 10.4 per cent and 9.7 per cent, respectively.

However, when the researchers pooled the data from both trials, they found the effect of low-dose supplementation was not significantly different on preterm birth compared to high-dose supplementation.