Vitamin D and Fertility: Does Your Level Matter?

Vitamin D appears on most prenatal and fertility supplement labels, and many people wonder whether a low level is holding them back. The research does link vitamin D status to pregnancy outcomes, but almost all of it is observational. Here is what that means for you.

How much vitamin D do you need?

The Institute of Medicine set the Recommended Dietary Allowance at 600 IU (15 mcg) a day for everyone aged 1 to 70, which covers people trying to conceive and pregnant people.¹ It found that intake meets the needs of at least 97.5% of the population, corresponding to a blood 25-hydroxyvitamin D level of at least 20 ng/mL (50 nmol/L).¹ The RDA assumes minimal sun exposure.¹

More is not automatically better. The same committee found that higher vitamin D levels were not consistently associated with greater benefit, saw U-shaped associations for some outcomes, with risks at both low and high levels, and concluded that the prevalence of vitamin D inadequacy in North America had been overestimated.¹

Vitamin D supplements, like all supplements, are not FDA-approved; by law the FDA does not approve supplements or their labels before sale.⁷

Vitamin D and fertility: the evidence at a glance

Does vitamin D affect your chances of getting pregnant?

Probably only modestly, and mainly if your level is low. The clearest natural-conception data come from 1,191 women aged 18 to 40 with one or two previous pregnancy losses, followed for up to six cycles while trying again.² About half (53%) had preconception levels below 75 nmol/L (30 ng/mL). Women with levels at or above that threshold were more likely to have a clinical pregnancy (adjusted RR 1.10) and a live birth (RR 1.15).² Each 25 nmol/L higher before conception was linked to a lower risk of pregnancy loss (RR 0.88).²

But sufficient levels did not shorten the time to conception: the fecundability odds ratio was 1.13 with a confidence interval crossing 1 (0.95 to 1.34).² And levels measured at 8 weeks of pregnancy were not linked to loss.²

A 2018 Harvard review summarized the wider picture: despite promising animal evidence, vitamin D does not appear to play an important role in human fertility in the absence of deficiency.⁶

What counts as a low level?

Thresholds differ, which makes headlines confusing. The Institute of Medicine based its RDA on a blood 25-hydroxyvitamin D level of at least 20 ng/mL (50 nmol/L), which it found meets the needs of nearly everyone.¹ The preconception study above used a higher cut-off, 75 nmol/L (30 ng/mL), to define “sufficient”.² So a result called insufficient in one study may be adequate by the IOM standard. Ask your doctor which threshold your lab uses.

Vitamin D and IVF

A 2018 meta-analysis pooled 11 cohort studies with 2,700 women undergoing assisted reproduction.³ Women replete in vitamin D were more likely to have a live birth (OR 1.33), a positive pregnancy test (OR 1.34) and a clinical pregnancy (OR 1.46) than women who were deficient or insufficient. There was no association with miscarriage.³ These were observational studies with varied populations and protocols, so they show a link, not proof that supplements improve IVF.³

Vitamin D during pregnancy

The Endocrine Society’s 2024 guideline suggests empiric vitamin D supplementation during pregnancy, meaning above the dietary reference intake and without testing, because of its potential to lower the risk of preeclampsia, intrauterine death, preterm birth, small-for-gestational-age birth and neonatal death.⁴ It adds that the optimal dose is unclear.⁴

A 2024 Cochrane review is more cautious. After removing 20 earlier studies over trustworthiness concerns, it was left with 10 trials and rated the evidence on preeclampsia, gestational diabetes and preterm birth as very uncertain.⁵

Should you test your vitamin D level?

Usually not. The Endocrine Society suggests against routine 25-hydroxyvitamin D testing in the absence of established indications, including in pregnancy.⁴ Your doctor may still test if they think you are at risk of deficiency.

What to do now

Make sure your daily total meets the 600 IU RDA, usually through a prenatal.¹ If you have a known deficiency, follow your doctor’s dose. Do not expect vitamin D to speed up conception if your level is normal; the evidence does not support that.

Check the vitamin D dose on your prenatal label. Every nutrient target is in prenatal nutrient doses. For the rest of your preparation, use our preconception checklist, read about DHA, and visit the Prepare hub.