Key messages
We found little to no difference in the number of women developing pre-eclampsia when they were given calcium during pregnancy, and we are uncertain about poor outcomes for mothers and babies.

Most participants started calcium in the middle 3 months of pregnancy, so we don't have information in this review about the effectiveness of calcium supplementation in very early pregnancy. We don't have information about women with enough calcium in their diet versus those who do not, nor those who are at high risk versus low risk of getting pre-eclampsia.

We found good evidence when we analysed only large studies, with more than 500 women. It is unlikely that further research would change the current evidence. Therefore, in future, research could focus on other ways to prevent blood pressure disorders during pregnancy.

Why is high blood pressure a problem during pregnancy?
High blood pressure in pregnancy is a leading cause of death and severe illness in mothers and babies. Pre-eclampsia is the most serious complication. It affects the placenta and can affect other organs, such as the kidneys, liver and brain. There is currently no treatment for pre-eclampsia apart from delivering the baby.

How might calcium help?
Some evidence suggests that calcium can lower blood pressure in people whose blood pressure is normal and in women who have had pre-eclampsia before, but other evidence does not support this. However, calcium is readily available, cheap and likely to be safe for mothers and babies. Calcium tablets are taken orally (swallowed). If calcium can prevent pre-eclampsia, it may reduce death and severe illness in mothers and babies.

What did we want to find out?
We wanted to know whether calcium is effective in preventing pre-eclampsia and other high blood pressure disorders when taken during pregnancy, and if it causes unwanted effects. We were also interested in whether calcium reduces the number of babies who die during or soon after birth, the number of mothers and babies who died or became ill, and babies who were born early.

What did we do?
We searched for studies that investigated calcium supplements during pregnancy. We used a checklist to make sure we only included studies that we could trust. We made judgements about the quality of the studies before comparing and summarising their results. Lastly, we rated our confidence in the findings.

What did we find?
We found 10 studies with 37,504 women that looked at the effects of calcium supplementation alongside standard care. Eight studies compared calcium supplementation to placebo (a dummy treatment), and two compared low-dose (500 mg daily) to high-dose (1500 mg daily) calcium supplementation. Studies took place worldwide, in high- and low-income countries. Some women in the studies had enough calcium in their diets and others did not. Some women were at high risk of pre-eclampsia and others weren't.

Calcium compared to placebo (8 studies, 15,504 women)
Evidence from 6 studies (15,364 women) showed that calcium may make little to no difference to pre-eclampsia compared to placebo. However, when we analysed only large studies with more than 500 women (4 studies, 14,730 women), we found strong evidence confirming that calcium makes little to no difference to pre-eclampsia compared to placebo.

Calcium probably results in little to no difference in the overall risk of a mother dying or developing severe complications of pre-eclampsia. It may result in little to no difference in death of the baby during pregnancy and early life.

We are very uncertain about the effect of calcium on the risk of mothers dying, on birth before 37 weeks, and also on unwanted effects.

Low- compared to high-dose calcium (2 studies, 22,000 women)
A lower dose of calcium may make little to no difference to pre-eclampsia compared to a higher dose. We are very uncertain about the effect of low-dose calcium on mothers dying compared to a higher dose. Taking a lower dose makes no difference to loss of the baby during pregnancy and early life, and probably makes little to no difference to birth before 37 weeks.

What are the limitations of the evidence?
Because most participants started calcium in the middle 3 months of pregnancy, we do not have information in this review about the effectiveness of calcium supplementation in very early pregnancy. This is the same for women who live in areas where people have enough calcium in their diet versus those who do not, and those who are at high risk versus low risk of getting pre-eclampsia.

How up to date is this evidence?
The evidence is current to January 2025.