Pregnancy · VITD-PREG-01 · v2
Across several trials, pregnant women who took vitamin D tended to develop pre-eclampsia less often, and the single largest trial [15] reported a clear reduction. But the trial built to test this question most rigorously, with blinding and a placebo, found no difference when everyone assigned to treatment was counted [2], and another trial also found no significant effect [8]. Many of the encouraging results come from women who were already vitamin D deficient or at high risk, or from studies that compared a high dose against a low dose rather than against no supplement at all [6], [13]. Several of the positive trials were also open-label, meaning everyone knew who was treated, which can color a judgement-based diagnosis like pre-eclampsia. The panel saw a consistent direction of benefit but judged the strongest evidence mixed and the supporting trials uneven, so it did not treat the effect as securely established.
Most included trials point the same direction: fewer pre-eclampsia cases among supplemented women, with the largest trial [15] reporting RR 0.36 (0.19-0.69), a recurrence trial [5] reporting the control group's risk 1.94 times higher (95% CI 1.02, 3.71), and a dose-comparison trial [6] reporting 1.2% versus 8.6% in the higher-dose arm. A prenatal screening-and-treatment programme [3] reported pre-eclampsia down by 60% in the screening arm. Against this, the only clearly double-blind placebo-controlled trial [2] was null in intention-to-treat (RR 0.97, 95% CI 0.61-1.53), with benefit emerging only in an adjusted cohort analysis by achieved vitamin D status (adjusted OR 0.28, 95% CI 0.10-0.96), and a nulliparous-women trial [8] found no significant difference (P=0.09). An observational study [19] links lower serum 25(OH)D to higher risk but tests no supplement. The contested point: The division is between a consistent protective direction across many trials and the fact that the single best-blinded, placebo-controlled trial [2] showed nothing in intention-to-treat. Whether the positive signal reflects a real supplementation effect or the weaker designs carrying it — open-label dose comparisons [6], [13] and deficient or recurrence-only populations [5], [6] — is the unresolved point.
All 19 papers behind this claim with their DOIs and what each does to it, the panel’s arbitration, the evidence gap map and the open research questions. Free, and sent to your inbox.
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One reviewer's only job is to attack the finding. The challenges below were weighed before the grade was set.
The only clearly double-blind, placebo-controlled trial powered on pre-eclampsia [2] was null in intention-to-treat (RR 0.97, 95% CI 0.61-1.53); its protective finding appeared only in a non-randomized cohort/sufficiency analysis that is confounded by baseline vitamin D status.
Two of the positive trials [6] and [13] are dose-vs-dose comparisons (high vs low vitamin D), not supplementation vs placebo, so they show a dose-response among supplemented women rather than that supplementation reduces risk against no supplementation.
Where it remains uncertain. The only clearly double-blind placebo-controlled trial [2] was null in intention-to-treat (RR 0.97, 0.61-1.53), and [8] was also null, while the positive trials skew open-label, dose-comparison, or deficient-population designs.
Aleatory
Low pre-eclampsia incidence in small trials such as [6] (total 4.3%) and [8] (two versus seven cases) leaves effect estimates sensitive to a handful of events.
Epistemic
The only clearly double-blind placebo-controlled trial [2] was null in intention-to-treat; no corpus trial combines placebo control, blinding, and a positive pre-eclampsia result.
Inferential
[6] and [13] compare higher versus lower dose, so a benefit there indexes dose-response among treated women, not supplementation versus no supplementation.
The load-bearing papers from a corpus of 19, each classified by what it does to the claim. Every DOI is taken from the corpus the claim was graded on.
Highest-quality double-blind ITT test; null, a key counterweight.
“There was no significant difference between treatment (N = 408) or control (N = 408) groups in the incidence of preeclampsia (8.08% vs. 8.33%, respectively; relative risk: 0.97; 95% CI, 0.61-1.53).”
Panel auditInternal validity: high · scope of inference: Women supplemented from 10-18 weeks; the cohort/observational association of achieved 25OHD ≥30 ng/ml is separate from the randomized contrast.Key limitations: active-comparator control not placebo · observational sub-finding not randomized
Positive, placebo-controlled, but restricted to recurrence (prior pre-eclampsia).
“The risk of preeclampsia for the control group was 1.94 times higher than that for the intervention group (95% CI 1.02, 3.71).”
Panel auditInternal validity: medium · scope of inference: Pregnant women with a history of preeclampsia (recurrence prevention), an Iranian single-centre cohort.Key limitations: small sample · recurrence-only population · CI lower bound near 1
Positive dose effect in deficient women; open-label, dose-vs-dose not placebo.
“the group 2 reported fewer pre-eclampsia events during the study period (8.6% versus 1.2%; p < 0.05)”
Panel auditInternal validity: medium · scope of inference: Vitamin D3-deficient pregnant women (<25 nmol/L) at a Saudi antenatal clinic; informs dose-response, not supplementation vs placebo.Key limitations: open-label · dose-dose contrast · deficient-only population · small
Supportive but composite outcome; pre-eclampsia not isolated.
“Forty-four percent patients in group A and 20·3% patients in group B developed preterm labour/pre-eclampsia/gestational diabetes.”
Panel auditInternal validity: low · scope of inference: Pregnant women at a tertiary centre in Delhi, India; a composite maternal-complication outcome rather than pre-eclampsia specifically.Key limitations: composite outcome · no placebo/blinding stated · small · unequal allocation
4 of 19 papers shown. The full review lists all of them, with the panel’s arbitration →
Claims are graded at each quarterly release, and re-graded between releases only when a pivotal trial lands. Every change is a new version of the claim.
Mixed RCT evidence leaning protective yet undercut by a null high-quality ITT trial and design weaknesses in the positive studies warrants a conservative Uncertain tier.
Release 2026-q4 re-graded every claim from its corpus under protocol v0.19. An earlier release's grade is shown for the record; it came from an earlier protocol and is not a movement.
Assumptions that carry weight but are not themselves established.
The panel records the gap that would most move the claim, and the study that would close it.
Blinded placebo-controlled trial in unselected pregnancy. The only blinded placebo trial [2] was null in ITT and the positive trials are open-label or dose comparisons, so the core causal question is untested under rigorous design in a general population. The panel rates this the highest-importance open gap for the claim, and names the study design that would close it: A large double-blind, placebo-controlled RCT in unselected pregnant women with pre-eclampsia as the pre-specified primary outcome, analyzed by intention-to-treat.
Open question: Does vitamin D supplementation reduce pre-eclampsia in unselected, vitamin D-replete pregnancies, not only in deficient or high-risk women? Why it is still open: Positive trials recruited deficient ([6]) or prior-pre-eclampsia ([5]) populations; the general-pregnancy claim is untested.
Priors is a sceptical first reader, not peer review, and not medical advice.
Release 2026-q4 · graded 26 Oct 2026 · protocol v0.19