Does Vitamin D Help You Get Pregnant? What Trials Measured

In the one large randomised trial that tested it directly, no. The SUNDRO trial randomised 630 women with 25(OH)D below 30 ng/mL who were starting IVF or ICSI to vitamin D3 or placebo. Cumulative clinical pregnancy per cycle was 37% in the treated group and 40% on placebo, risk ratio 0.91 (95% CI 0.75 to 1.11). Supplementation did not improve the outcome, and the direction of the point estimate was, if anything, slightly the wrong way.

Deficiency is still worth taking seriously. It is common in India, and correcting it has well-established value for bone and muscle health. What has not been shown is that correcting it improves the chance of conceiving. Both of those are true at once, and most pages on this topic print only the first.

Yes - and this matters, because live birth is the outcome that counts and most supplement trials stop at a lab value. SUNDRO pre-specified live birth and miscarriage as secondary outcomes alongside its primary outcome of cumulative clinical pregnancy per cycle. The primary outcome was negative, and the trial reported no clinical situation that benefited.

The trial then went looking. Exploratory subgroups by body mass index, age, indication for IVF, ovarian reserve, the interval between the dose and the start of the cycle, and baseline 25(OH)D level all failed to identify any group that did better on vitamin D. That is a meaningful negative: it is the answer to "but surely it helps the women who are most deficient", and the answer was no.

On the male side the picture is the same shape: a randomised trial in 330 infertile men found no change in semen quality, with one unreplicated subgroup exception. What the trials found for men is set out in vitamin D and sperm, because the outcomes measured there are semen parameters and DNA fragmentation rather than IVF success.

Because association and intervention are different questions, and on this topic they give different answers. A 2018 meta-analysis found that women with deficient vitamin D had a significantly lower probability of live birth after IVF or ICSI, risk ratio 0.74 (95% CI 0.58 to 0.90). That is a real finding and it is why this idea took hold.

Two things constrain it. Every study pooled in that analysis was observational - a cohort, not a trial - so nobody was randomised and the groups differed in more than their vitamin D. And the live birth estimate came from only 3 of the studies; the 9 studies often quoted were the ones contributing to the clinical pregnancy analysis, which showed no significant difference. Worth noting too that those authors concluded vitamin D "should be supplied" to deficient women - a recommendation their own observational design could not support, and one the later trial did not bear out.

The likeliest explanation is confounding. A low vitamin D level travels with other things - less time outdoors, higher body weight, chronic illness, socioeconomic position - and some of those independently affect the chance of a live birth. When a trial removes the confounding by randomising, the effect goes away. That pattern has repeated across fertility supplements.

This is where the literature is easiest to misread. A 2024 umbrella review of meta-analyses of randomised trials reported that vitamin D was associated with higher clinical pregnancy in women with PCOS and/or using assisted reproduction, odds ratio 1.49 - but graded that finding at very low certainty, and found that vitamin D did not increase biochemical pregnancy rates. Its overall conclusion was that the available evidence is insufficient to recommend nutrient supplementation to improve female infertility.

The other problem with the PCOS trials is what they measure. Many use metabolic surrogates - insulin resistance, lipid profiles, cycle regularity - rather than conception. An improvement in a metabolic marker is not a pregnancy, and a trial that never measured conception cannot tell you whether conception changed.

Common enough that a low result is close to unremarkable. A meta-analysis of 65 South Asian studies covering 44,717 participants put pooled deficiency in India at 67% (95% CI 61 to 73), with a regional weighted mean level of 19.15 ng/mL. Deficiency was more prevalent in females than males across the region. Living at a sunny latitude does not protect a population from this.

High prevalence is a reason to treat deficiency as a general health problem. It is not evidence that supplementation improves fertility - the trial tested exactly that and came back negative. The slide from "deficiency is widespread" to "supplementation improves fertility" is the commonest error on this subject, and the two claims need separate evidence.

The test measures serum 25-hydroxyvitamin D, is cheap and is widely available in India. What it will not do is explain subfertility or predict whether treatment will work. Whether to check it is reasonably framed as a general health question rather than a fertility one.

A low result would change three things, none of them a fertility intervention:

• It identifies a deficiency with established consequences for bone and muscle health, which a prescriber can treat on that basis.
• It gives a baseline, so a repeat level shows whether what was prescribed is being absorbed.
• It rules the question out, so attention moves to the things that do change outcomes: ovarian reserve testing, tubal and uterine assessment, a semen analysis for the partner, and the choice and timing of treatment.

Thresholds are not agreed, which is worth knowing before a number worries you. SUNDRO enrolled women below 30 ng/mL and called that low; the male analyses used below 20 ng/mL as deficient. Different labs and guidelines draw the line in different places, and no threshold has been validated against fertility outcomes.

Not on the basis of this page. Stopping a prescription is a conversation with whoever wrote it, and there are good reasons to be on vitamin D that have nothing to do with fertility. The umbrella review found no indication that these nutrients pose a risk of significant harm, and no effect on miscarriage rates, so there is nothing here that argues for stopping.

What is worth asking is what the prescription is for. If the answer is "your level was low" or relates to bone health or pregnancy, that is defensible and separately supported. If the answer is "to improve your chances of conceiving" or "to improve your IVF success rate", that is the specific claim the 630-woman trial tested and did not support. Dosing is a prescriber's decision and this page deliberately does not give one - the regimens quoted here were research protocols, not advice.

Set out plainly, because the gap between what has been measured and what gets claimed is wide here:

• That vitamin D supplementation improves clinical pregnancy or live birth. The randomised trial measured this in 630 women with low levels and did not find it.
• That women who are most deficient are the ones who benefit. SUNDRO tested that directly in its subgroup by baseline 25(OH)D and found no group that benefited.
• That vitamin D raises AMH or improves ovarian reserve. We could not retrieve a randomised trial measuring this outcome, so it is an open question rather than a finding - and an untested claim should not be presented as one.
• That vitamin D prevents miscarriage. The umbrella review found no effect on miscarriage rates, at low to very low certainty.
• That the PCOS clinical pregnancy signal is reliable. It was graded very low certainty, and many PCOS trials measure metabolic markers rather than conception.
• That there is a vitamin D level below which a woman should be treated for fertility reasons. No threshold has been validated against fertility outcomes.
• That a low level explains why a cycle failed. Two in three Indian adults are deficient by the pooled prevalence estimate, so it is far too common to serve as an explanation for one person's outcome.

If you have been taking vitamin D for months hoping it was the thing that would finally work, it is reasonable to feel deflated reading this. The more useful reading is that it clears the question off the list - what changes outcomes is usually a diagnosis and the right treatment for it, not a supplement.