Vitamin D and Fertility: An Association, Not a Reason to Supplement

Q: Should everyone trying to conceive take vitamin D?

**A1**: Not for the purpose of improving fertility outcomes — the evidence does not support that. Whether vitamin D matters for general and bone health is a separate question. If you are concerned about deficiency, have your level measured.

Q: How much should I take?

**A2**: General intake targets and upper limits exist, but no optimal dose for fertility has been established. Measure your level, then decide with your doctor or pharmacist. Because it accumulates, avoid self-directed high doses.

Q: I read that women with enough vitamin D conceive more easily.

**A3**: That observation has been reported in several studies. But it is an association, not causation. People with low levels differ in other ways too, and whether raising the level improves outcomes has to be tested separately. That testing has not been completed.

Q: Is sunlight alone enough?

**A4**: It depends on season, location, time outdoors and skin tone. If you spend little time outside or live somewhere with limited winter sun, deficiency is more likely. Measuring is the way to know.

Vitamin D is essential for bone health, and in recent years its relationship with reproductive function has drawn attention. Many people trying to conceive wonder whether they should be taking it.

Here is the conclusion up front. Multiple studies observe that women with sufficient vitamin D levels have better IVF outcomes. That does not mean taking a supplement improves those outcomes. This distinction is the single biggest source of confusion on this topic.

Why vitamin D and fertility get linked

Vitamin D is converted to an active form in the body and acts somewhat like a hormone. Vitamin D receptors have been found not only in the kidney and gut but in the ovary and uterus. From there comes the hypothesis that it plays a role in sex hormone production, follicular development, and the state of the endometrium.

Epidemiological observations that vitamin D deficiency is more common among women with infertility have added to the interest.

What has been observed

Association with IVF outcomes

A meta-analysis pooled 15 cohort studies totalling 3,711 women undergoing IVF, dividing them by blood level into replete (30 ng/mL or above), insufficient (21-29 ng/mL) and deficient (below 20 ng/mL). Compared with the deficient group, the replete group had higher rates of (PMID: 32791871):

Live birth (OR 1.74, 95% CI 1.31-2.31)
Biochemical pregnancy (OR 1.43, 95% CI 1.06-1.95)
Ongoing pregnancy (OR 1.29, 95% CI 1.02-1.64)

Implantation and miscarriage rates did not differ. Notably, when only donor-oocyte cycles were considered, no significant differences were found.

A separate pooling of cohort studies (nine for clinical pregnancy) also reported a lower live birth rate in the deficient group (risk ratio 0.74, 95% CI 0.58-0.90), with no significant difference in clinical pregnancy rate (PMID: 29426322).

This is the crux

Both of the above are pooled cohort studies — observations, not trials of supplementation.

Blood vitamin D level reflects time spent outdoors, body size, diet, season, latitude, and underlying health. People with low levels differ from people with high levels in many other ways too. So "people with higher levels do better" does not establish that raising the level improves outcomes. The disappearance of the difference in donor-oocyte cycles points the same way: the egg side alone does not explain it.

The authors of the meta-analysis state that many demographic, geographic and clinical factors related to vitamin D status would need to be accounted for before concluding that better results are due to higher vitamin D levels.

Trials that administered vitamin D and compared pregnancy outcomes have not produced consistent results, and there is currently not enough evidence to recommend vitamin D for the purpose of improving fertility outcomes.

What has been reported in men

A pooled analysis of 18 observational studies found higher serum 25(OH)D3 in fertile men than in infertile men, and an association between serum 25(OH)D and sperm motility and progressive motility (PMID: 31561004).

This too is a pooling of observational studies. The authors state explicitly that heterogeneity between the included studies was substantial and that the results warrant caution.

When deficiency is more likely

Vitamin D is produced in the skin through sun exposure and also obtained from food. Limited time outdoors, thorough sun protection, and a diet low in fish all make deficiency more likely. In winter and at higher latitudes, production from sunlight falls regardless.

If you are concerned, measuring your blood 25(OH)D level is more reliable than guessing.

If you supplement

From food: oily fish such as salmon and tuna, and mushrooms
From sunlight: depending on season and location, brief exposure of arms or legs is enough. You do not need to burn
From supplements: do not continue high doses on your own judgment. Vitamin D is fat-soluble and accumulates; excess can cause hypercalcaemia (nausea, excessive urination, weakness, impaired kidney function). If deficiency is suspected, measure your level first and decide the dose with your doctor

Frequently Asked Questions

Q1: Should everyone trying to conceive take vitamin D?

A1: Not for the purpose of improving fertility outcomes — the evidence does not support that. Whether vitamin D matters for general and bone health is a separate question. If you are concerned about deficiency, have your level measured.

Q2: How much should I take?

A2: General intake targets and upper limits exist, but no optimal dose for fertility has been established. Measure your level, then decide with your doctor or pharmacist. Because it accumulates, avoid self-directed high doses.

Q3: I read that women with enough vitamin D conceive more easily.

A3: That observation has been reported in several studies. But it is an association, not causation. People with low levels differ in other ways too, and whether raising the level improves outcomes has to be tested separately. That testing has not been completed.

Q4: Is sunlight alone enough?

A4: It depends on season, location, time outdoors and skin tone. If you spend little time outside or live somewhere with limited winter sun, deficiency is more likely. Measuring is the way to know.

Summary

There is an accumulation of observational evidence linking vitamin D status and fertility outcomes. Pooled cohort studies of 3,711 women undergoing IVF report higher live birth rates in the replete group.

At the same time, no improvement from taking a supplement has been demonstrated. Association and causation are not the same thing.

Vitamin D is a nutrient your body needs, so if you are deficient there is reason to correct it. But the reason is that you are deficient — not that it will help you conceive. Start by measuring.

References

Women's vitamin D levels and IVF results: a systematic review of the literature and meta-analysis, considering three categories of vitamin status (replete, insufficient and deficient). Hum Fertil (Camb). 2022. PMID: 32791871 (a pooling of 15 cohort studies, not an intervention trial)
Whether vitamin D was associated with clinical outcome after IVF/ICSI: a systematic review and meta-analysis. Reprod Biol Endocrinol. 2018. PMID: 29426322 (a pooling of cohort studies)
The association between serum vitamin D, fertility and semen quality: A systematic review and meta-analysis. Int J Surg. 2019. PMID: 31561004 (a pooling of 18 observational studies)