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Not medical advice. The Atlas is not a substitute for a clinician who can examine you.

Calcium

Bone Mineral and Preeclampsia Shield

Evidence grade 3 of 5, Mixed. Read how the Atlas grades evidence.

10 human trials, about 27 studies, 2024 participants NEJM trials: ~22,000 women (India + Tanzania). The written rationale for this grade is still being prepared.

FDA
Regulated as a dietary supplement; an essential nutrient with an RDA
WADA
Not prohibited

Reference only. Not medical advice.

What can go wrong

Calcium is generally well tolerated within the tolerable upper intake level, which the NIH Office of Dietary Supplements sets at 2,500 mg per day for ages 19 to 50 and 3,000 mg for ages 14 to 18. Very high intakes, mainly from supplements, are associated with kidney stones and can interfere with the absorption of other minerals. Calcium and iron each reduce the other's absorption, so when both are advised they are typically spaced a few hours apart, which is a real practical hurdle. Any pre-eclampsia-prevention benefit is described mainly in people with low dietary calcium, and the newest large-trial evidence questions even that. Whether to supplement calcium in pregnancy, and how to balance it against iron and other prenatal nutrients, is a decision to make with an obstetric provider rather than on your own.

Calcium builds fetal bone and teeth, and the pregnancy requirement is the same as for other adults, about 1,000 mg per day for ages 19 to 50, yet it is a common shortfall and prenatal vitamins usually carry little of it. Its most-discussed role has been reducing pre-eclampsia risk in populations with low dietary calcium, where WHO has recommended 1,500 to 2,000 mg per day since 2011. That case is now contested: two large 2024 trials in India and Tanzania found a simpler 500 mg dose non-inferior to the high dose, and a December 2025 Cochrane update found that once analysis is limited to large, higher-quality trials, calcium shows little to no effect on pre-eclampsia. Bone-mineral need is not in dispute. Decisions about supplementation in pregnancy belong with an obstetric provider.

A bone mineral most people fall short on, and one of the few supplements with strong trial evidence for lowering preeclampsia risk where dietary calcium is low.

What people take it for

These are the reasons people give for taking it. They are not effects it has been shown to have.

  • Building fetal bone and teeth
  • Meeting the pregnancy calcium requirement when diet falls short
  • Protecting maternal bone stores
  • Reducing pre-eclampsia risk in populations with low dietary calcium, a use WHO recommends but that newer large-trial evidence has called into question

What people get wrong

  • Often saidMy prenatal vitamin covers my calcium needs.

    ActuallyPrenatal supplements usually contain relatively little calcium, and many people fall short of the roughly 1,000 mg daily target for ages 19 to 50. Calcium generally comes from diet, such as dairy or fortified alternatives, rather than the prenatal alone, per the NIH Office of Dietary Supplements.

  • Often saidCalcium supplements are a proven way to prevent pre-eclampsia for everyone.

    ActuallyThe benefit has always been described mainly in populations with low dietary calcium, not universally, and a December 2025 Cochrane update found that once the analysis is limited to large, higher-quality trials, calcium shows little to no effect on pre-eclampsia. WHO still recommends it in low-intake settings, but the evidence is now contested rather than settled.

  • Often saidYou need the full high dose (1,500-2,000 mg) for any pre-eclampsia benefit.

    ActuallyTwo large 2024 NEJM trials in India and Tanzania found a single 500 mg dose non-inferior to 1,500 mg for pre-eclampsia, so where a benefit exists in low-intake populations, the simpler low dose appears to capture most of it.

Who profits, and who pushes back

  • World Health Organization

    championsAntenatal-care guidelines (since 2011)

    In populations with low dietary calcium intake, 1,500 to 2,000 mg of daily calcium is recommended for pregnant women to reduce the risk of preeclampsia and its complications.

  • Hofmeyr et al. / Cochrane reviewers

    championsSystematic-review evidence

    Calcium supplementation above 1,000 mg per day is associated with roughly a 55% lower risk of preeclampsia, with even larger reductions in high-risk women and those with low dietary calcium.

  • 2024 NEJM low-dose trial investigators

    endorsesIndia + Tanzania, ~22,000 women

    A 500 mg daily dose was non-inferior to 1,500 mg for preeclampsia risk, which could simplify and expand calcium supplementation programs in low-intake settings.

  • NIH Office of Dietary Supplements

    caveatsAuthoritative reference

    Calcium supplementation of 1,500 to 2,000 mg/day may reduce preeclampsia risk in those with insufficient dietary calcium; the benefit is tied to low baseline intake rather than universal.

Where the research comes from

Most of the published research on this comes from one place. 55% of it is from Low dietary intake. Findings that no unrelated group has reproduced are weaker than the study count makes them look.

Share of published research by country of origin.
PlaceShare of published research
Low dietary intake55%
Adequate intake45%

Research published from: Global.

How we got here

Interest in calcium and blood pressure in pregnancy grew from observations in the Americas that Indigenous Maya communities in Guatemala, with calcium-rich lime-treated maize diets, had low rates of the hypertensive disorder later called pre-eclampsia. Trials through the 1990s and 2000s, many in Latin America, suggested supplementation cut that risk, most visibly where dietary calcium was low. WHO recommended high-dose supplementation in low-intake populations from 2011 and revalidated it in 2018. Uptake stayed thin: three-times-daily dosing kept apart from iron is hard to sustain and costs more than iron-folic acid alone, and by one 2025 count only 29 of 92 countries with antenatal guidelines recommended it. Two 2024 NEJM trials testing a single 500 mg dose suggested a far simpler regimen, and a December 2025 Cochrane update, after setting aside several trials over data-integrity concerns, questioned whether the pre-eclampsia benefit holds at all.

  1. 1980sLow pre-eclampsia rates in high-calcium Maya communities in Guatemala spur interest in calcium and blood pressure in pregnancy
  2. 1990s-2000sTrials, many in Latin America, associate calcium supplementation with lower pre-eclampsia risk, most clearly where dietary calcium is low
  3. 2011WHO recommends 1,500-2,000 mg/day calcium in low-intake populations to reduce pre-eclampsia risk
  4. 2018WHO revalidates the calcium recommendation
  5. 2024Two large NEJM trials in India and Tanzania find a 500 mg dose non-inferior to 1,500 mg for pre-eclampsia
  6. 2024A BJOG reanalysis by Wright and colleagues reports the three largest trials pool to a null result (RR 0.92)
  7. 2025Updated Cochrane review finds high-certainty evidence of little to no pre-eclampsia effect once analysis is limited to large trials

At a glance

Pregnancy RDA
1,000 mg/day (19-50y)
WHO supplement
1,500-2,000 mg/day
Low-dose (new trials)
500 mg non-inferior
UL (19-50y)
2,500 mg/day

What this entry does not yet cover

What follows is unfinished work on our side, not a finding about Calcium. A short entry is not a safe entry.

  • Cost and market. We have not documented what it sells for or who sells it.
  • Supply chain. We have not mapped where it is made.
  • Who uses it. We have not documented the user groups.
  • What could change. No forward-looking summary yet.
  • How it compares. We have not set it against the alternatives.
  • Children and teenagers. We have not checked what is known about younger people.

Sources