First, quick context.

What Is PCOS?

Polycystic ovary syndrome (PCOS) is one of the most common hormonal conditions in reproductive-aged women.

It’s typically diagnosed when at least two of these show up:

  • High androgens (think acne, excess hair growth)
  • Irregular or absent ovulation
  • Polycystic-appearing ovaries on ultrasound

It affects roughly 5–20% of women worldwide.

And it’s one of the leading causes of infertility.

Women with PCOS who undergo IVF often face higher risks of:

  • Fertilization failure
  • Early pregnancy loss
  • Miscarriage
  • Gestational diabetes
  • Preterm birth

So when researchers noticed something else common in PCOS…

Low vitamin D…

The lightbulb went on.

The Vitamin D + PCOS Hypothesis

Vitamin D deficiency is common in the general population.

But in women with PCOS?

It’s even more common.

Severe deficiency (<10 ng/mL) has been reported in:

  • 44% of women with PCOS
  • vs 11% without PCOS

Add to that:

  • Vitamin D receptors exist in the ovaries
  • In the endometrium
  • In the placenta

Which means biologically, it could influence:

  • Egg development
  • Uterine lining receptivity
  • Implantation

Smaller studies and meta-analyses hinted that vitamin D might improve clinical pregnancy rates.

But the biggest prior IVF study?
One giant 600,000 IU bolus dose before IVF.

Result: No improvement.

So researchers designed a better test.

The Study: Big, Rigorous, No Guesswork

This wasn’t a “20 people in one clinic” experiment.

It was:

  • Multicenter
  • Randomized
  • Double-blind
  • Placebo-controlled
  • 865 women with PCOS
  • Daily vitamin D (4000 IU) for up to 90 days before IVF

Gold standard design.

They measured what actually matters:

Live birth rate.

Not hormone levels.
Not “chemical pregnancy.”
Not surrogate markers.

Actual babies.

What Happened?

Vitamin D levels went up.

Live birth rates?

Did not.

There was no statistically significant difference between:

  • Women who took 4000 IU daily
  • Women who took placebo

And that held true:

  • Regardless of baseline vitamin D levels
  • Across different IVF protocols
  • Across intention-to-treat and per-protocol analyses

In short:

More vitamin D in the bloodstream
≠
More babies.

But Wait — Why Didn’t It Work?

There are a few possibilities.

1) Vitamin D Might Be a Marker — Not a Cause

Just because low vitamin D is common in PCOS doesn’t mean it causes infertility.

It may simply travel alongside:

  • Insulin resistance
  • Higher BMI
  • Less sun exposure
  • Metabolic dysfunction

If it’s a passenger, not the driver, supplementation won’t fix the engine.

2) Maybe the Dose Wasn’t High Enough (For Some)

Some earlier data suggested IVF outcomes improved when vitamin D levels hit ≥38 ng/mL.

In this trial?

Only about 25% of women reached that level.

So it’s possible only a subgroup would benefit.

But that wasn’t proven.

3) Genetics May Matter

Vitamin D metabolism varies by person.

Differences in:

  • Vitamin D binding protein
  • Receptor activity
  • Enzyme function

Could affect who responds.

The trial wasn’t designed to personalize dosing.

4) IVF Success Rates Were Already High

Interestingly, the placebo group had a 50% live birth rate — higher than expected.

When outcomes are already strong, it’s harder to show added benefit.

Important: This Doesn’t Mean Vitamin D Is Useless

Vitamin D still matters for:

  • Bone health
  • Immune function
  • Metabolic regulation
  • Insulin sensitivity

And in PCOS specifically, some studies show it can:

  • Lower fasting glucose
  • Improve insulin resistance
  • Improve lipid markers
  • Increase endometrial thickness

But improving lab markers
doesn’t automatically mean improving live birth rates.

And that distinction matters.

What This Means for Women With PCOS Doing IVF

If you’re vitamin D deficient:

Correcting it is still reasonable for overall health.

But based on this large, well-designed trial:

Taking 4000 IU daily for 90 days before IVF
did not significantly increase live birth rates.

Could there be a small benefit?

Possibly.

The study couldn’t rule out a modest effect.

But there was no meaningful boost.

What We Still Don’t Know

Future research needs to answer:

  • Does longer supplementation help?
  • Should vitamin D continue through embryo transfer?
  • Does severe deficiency (<10 ng/mL) respond differently?
  • Is there a dose–response curve?
  • Do genetics change outcomes?

For now, though:

Vitamin D is not a proven IVF success booster in PCOS.

The Bottom Line

The theory made sense.

Vitamin D affects metabolism.
PCOS involves metabolic dysfunction.
Vitamin D receptors exist in reproductive tissue.

But in a large, rigorous trial?

Higher vitamin D levels
did not translate into higher live birth rates.

Sometimes biology is compelling.

And sometimes it just doesn’t move the outcome that matters.

Babies > biomarkers.

Hu KL, Liao T, Wu Q, Ma X, Cao Y, Tan J, Tian L, Wang J, Yin J, Liu Y, Zhao J, Zhao S, Li M, Cai L, Liu FT, Gan K, Xu Y, Wang Y, Cai J, Zheng B, Ma Y, Ma Q, Zheng J, Pu X, Zhang H, Hao C, Xie Q, Zhang C, Jiang L, Zhang S, Yan L, Meng Q, Li W, Mol BW, Li R, Wang R, Zhang D; VitD-PCOS trial group. Vitamin D supplementation before in vitro fertilisation in women with polycystic ovary syndrome: multicentre, double blind, placebo controlled, randomised clinical trial. BMJ. 2026 Feb 17;392:e087438. doi: 10.1136/bmj-2025-087438. PMID: 41702641; PMCID: PMC12910436.