Key messages
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Evidence from one study (1355 women) shows that extra calcium before pregnancy may result in little to no difference to pre-eclampsia (a complication of pregnancy) or pregnancy loss, and pre-eclampsia. Evidence from 633 women who got pregnant during the study showed similar results except for a slight reduction in pre-eclampsia or pregnancy loss.
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The study did not provide evidence for unwanted effects of calcium or death of the baby during pregnancy and early life.
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We need more research to decide whether calcium supplements started before pregnancy help women avoid high blood pressure, pre-eclampsia and other complications. Research should also address how acceptable women find calcium supplements.
Why is high blood pressure a problem in pregnancy, and how could calcium help?
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 is diagnosed when the pregnant woman develops high blood pressure with signs of damage to the placenta and other organs such as the kidneys or liver. There is currently no treatment for pre-eclampsia apart from delivering the baby. Calcium may help to lower blood pressure. Many people, especially in low-income countries, have insufficient calcium in their diets, so giving calcium supplements before pregnancy may prevent pre-eclampsia and could save many mothers and babies.
What did we want to find out?
We wanted to know if giving calcium supplements to women before they get pregnant could help prevent high blood pressure, pre-eclampsia and other serious health problems for the mother and baby. We were also interested in any unwanted effects of calcium supplements.
What did we do?
We searched for studies that compared giving calcium supplements before pregnancy with placebo (a dummy medicine) or normal care. Women in the studies had to be planning a pregnancy. They could be any age and live anywhere in the world. Their normal diets could include any amount of calcium, and they could be at any risk of high blood pressure. Studies could give any dose of calcium for any length of time, as long as supplements started before pregnancy. We looked at the evidence for all the women in the study, and only for women who became pregnant during the study.
We compared and summarised the studies' results, and rated our confidence in the evidence, based on factors such as study methods and sizes.
What did we find?
We found one study with 1355 women. They had all had pre-eclampsia in their most recent pregnancy, so were at high risk of getting it again. They were given extra calcium or a placebo tablet daily until 20 weeks of pregnancy. After 20 weeks, all women were switched to extra daily calcium until they gave birth. They lived in Argentina, South Africa, and Zimbabwe.
Calcium supplements before pregnancy may make little to no difference to:
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pre-eclampsia or pregnancy loss;
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pre-eclampsia;
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pregnancy loss; and
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delivery of the baby before 37 weeks.
We are not sure about the effect of extra calcium before pregnancy on death of the mother, death or severe illness of the mother and stillbirth.
The results for 633 women who got pregnant during the study are very similar, except that there may be a slight reduction in pre-eclampsia or pregnancy loss.
There was no evidence about unwanted effects, death of the baby during pregnancy and early life, death of the newborn baby, death of newborns in the first 7 days of life, or death or severe illness of newborns.
What are the limitations of the evidence?
Our confidence in the evidence from this study is limited because nearly a third of the women included were not followed up or withdrew from the study. We do not know whether these women became pregnant and what their outcomes were.
How up to date is this evidence?
The evidence is current to January 2025.
Read the full abstract
The hypertensive disorders of pregnancy include pre-eclampsia, gestational hypertension, chronic hypertension, and undefined hypertension. Pre-eclampsia is considerably more prevalent in low-income than in high-income countries. One possible explanation for this discrepancy is dietary differences, particularly calcium deficiency. Calcium supplementation in the second half of pregnancy reduces the serious consequences of pre-eclampsia, but has limited effect on the overall risk of pre-eclampsia. It is important to establish whether calcium supplementation before, and in early pregnancy (before 20 weeks' gestation) has added benefit. Such evidence could count towards justification of population-level interventions to improve dietary calcium intake, including fortification of staple foods with calcium, especially in contexts where dietary calcium intake is known to be inadequate. This is an update of a review first published in 2017.
Objectives
To assess the effects of calcium supplementation commenced before pregnancy on hypertensive disorders of pregnancy and related maternal and neonatal outcomes.
Search strategy
We searched CENTRAL, MEDLINE, Embase, CINAHL, Portal Regional BVS-Lilacs, Scopus, Web of Science, WHO ICTRP and ClinicalTrials.gov on 7 January 2025 and searched reference lists of retrieved trials and relevant systematic reviews.
Selection criteria
Eligible studies were randomised controlled trials (RCT) of calcium supplementation, including women not yet pregnant, or women in early pregnancy. Cluster-RCTs, quasi-RCTs, and trials published as abstracts were eligible, but we did not identify any.
Data collection and analysis
Two review authors independently assessed trials for inclusion and risk of bias, extracted data, and checked them for accuracy. They assessed the quality of the evidence for key outcomes using the GRADE approach.
Main results
Calcium versus placebo
We included one study (1355 women), which took place across multiple hospital sites in Argentina, South Africa, and Zimbabwe. Most analyses were conducted only on 633 women from this group who were known to have conceived, or on 579 who reached 20 weeks' gestation; the trial was at moderate risk of bias due to high attrition rates pre-conception. Non-pregnant women with previous pre-eclampsia received either calcium 500 mg daily or placebo, from enrolment until 20 weeks' gestation. All participants received calcium 1.5 g daily from 20 weeks until birth.
Primary outcomes: calcium supplementation commencing before conception may make little or no difference to the risk of pre-eclampsia (69/296 versus 82/283, risk ratio (RR) 0.80, 95% confidence interval (CI) 0.61 to 1.06; low-quality evidence). For pre-eclampsia or pregnancy loss or stillbirth (or both) at any gestational age, calcium may slightly reduce the risk of this composite outcome, however the 95% CI met the line of no effect (RR 0.82, 95% CI 0.66 to 1.00; low-quality evidence). Supplementation may make little or no difference to the severe maternal morbidity and mortality index (RR 0.93, 95% CI 0.68 to 1.26; low-quality evidence), pregnancy loss or stillbirth at any gestational age (RR 0.83, 95% CI 0.61 to 1,14; low-quality evidence), or caesarean section (RR 1.11, 95% CI 0.96 to 1,28; low-quality evidence).
Calcium supplementation may make little or no difference to the following secondary outcomes: birthweight < 2500 g (RR 1.00, 95% CI 0.76 to 1.30; low-quality evidence), preterm birth < 37 weeks (RR 0.90, 95% CI 0.74 to 1.10), early preterm birth < 32 weeks (RR 0.79, 95% CI 0.56 to 1.12), and pregnancy loss, stillbirth or neonatal death before discharge (RR 0.82, 95% CI 0.61 to 1.10; low-quality evidence), no conception, gestational hypertension, gestational proteinuria, severe gestational hypertension, severe pre-eclampsia, severe pre-eclamptic complications index. There was no clear evidence on whether or not calcium might make a difference to perinatal death, or neonatal intensive care unit admission for > 24h, or both (RR 1.11, 95% CI 0.77 to 1.60; low-quality evidence).
It is unclear what impact calcium supplementation has on Apgar score < 7 at five minutes (RR 0.43, 95% CI 0.15 to 1.21; very low-quality evidence), stillbirth, early onset pre-eclampsia, eclampsia, placental abruption, intensive care unit admission > 24 hours, maternal death, hospital stay > 7 days from birth, and pregnancy loss before 20 weeks' gestation.
Authors' conclusions
When all randomised women are considered, calcium commenced before pregnancy may result in little to no difference in pre-eclampsia or pregnancy loss, and pre-eclampsia. When only pregnant women are considered, calcium may result in little to no difference in pre-eclampsia but may result in a slight reduction in pre-eclampsia or pregnancy loss. No trials measured perinatal loss.
The evidence is drawn from one trial of calcium supplementation that commenced before and continued into the first half of pregnancy. Current evidence neither supports nor refutes the routine use of calcium supplementation commencing before conception.
Funding
This review was funded in part by the World Health Organization.
Registration
Updated protocol (2025): PROSPERO: CRD420250649571
Review update (2019): DOI: 10.1002/14651858.CD011192.pub3
Original review (2017): DOI: 10.1002/14651858.CD011192.pub2
