Research debunks view that calcium lowers pre-eclampsia risk during pregnancy

Calcium supplementation has no effect on pre-eclampsia risk, a serious pregnancy complication, according to a new review. It challenges earlier research that calcium plays a role in preventing hypertensive disorders. The authors argue that previous findings were based on small and unreliable studies.

Researchers at Stellenbosch University in South Africa studied data from 37,504 participants in 10 randomized controlled trials and strongly concluded that calcium, neither at high nor low doses, reduces the risk of pre-eclampsia.

They warn of the risks of small studies and publication bias that exaggerated the earlier assumption. 

“After excluding unreliable studies, we found that the supposed benefit of calcium supplementation on pre-eclampsia disappeared. We now show that calcium supplementation does not prevent pre-eclampsia, based on large, reliable trials,” says review co-author professor Catherine Cluver.

Mixed views

Pre-eclampsia can impact women in the second half of their pregnancy and can be fatal or cause lifelong complications for the mother and the baby. Symptoms include high blood pressure and organ damage. 

Mothers with this condition can only be cured by delivering the baby, even if it means preterm delivery.

Until now, calcium supplementation was considered to be a preventative measure, and current WHO guidelines recommend daily calcium supplementation in populations with low intake levels, note the researchers. 

However, increasing evidence shows that the effectiveness of calcium supplementation has been mixed. The new review raises further doubts.

“By applying rigorous and transparent review methods, we found no meaningful difference in key outcomes such as pre-eclampsia, maternal death, preterm birth, or neonatal mortality,” says review author Anke Rohwer. 

“Many of the older studies were of very low quality, and re-evaluating them was essential to ensure that current guidance is based on reliable, up-to-date evidence.”

Small-trial effects and publication bias

While the new review, published in Cochrane Database of Systematic Reviews, examined results from large trials, a previous review update included additional studies. The review authors excluded these because they failed to meet the eligibility criteria, lacked trustworthiness, or had methodological flaws.

Previous analyses that included these studies made stronger claims about calcium’s protection.

“Our updated analysis shows that once you account for issues like small-trial effects and publication bias, the evidence supporting calcium supplementation to prevent pre-eclampsia simply doesn’t hold up,” explains Cluver. 

“This marks a major shift from earlier reviews, and it’s crucial that clinicians and policymakers understand how much the evidence base has changed. This review highlights the importance of applying trustworthiness checks to primary research. Unreliable trials can skew the results of systematic reviews and distort scientific consensus.”

Previous research on calcium also found no evidence that it increases the long-term risk of dementia in older women. This debunked previous theories that the mineral supplements could impact cognitive health.

Additionally, research linked alcohol and red or processed meat to increased colorectal cancer risk, while dietary calcium can offer significant protective benefits.