Vitamin D and fertility

What does vitamin D do?

Your body makes vitamin D when sunlight reaches your skin, and you get some from food. It supports calcium use and the development of your baby's skeleton. Needs are the same whether you are trying to conceive or already pregnant, so building a habit now carries straight into pregnancy.

What does the research say about fertility?

Correcting a true deficiency is worthwhile for your health. Evidence that taking extra vitamin D improves the chance of conceiving is lacking, and it is best not to expect a fertility benefit from a supplement. A large umbrella review of nutrient supplements and female infertility found that the apparent benefits for many nutrients rested on very low-certainty evidence, and it did not support supplements as a general fertility treatment.

For pregnancy outcomes, a 2024 Cochrane update found considerable uncertainty about several outcomes, including pre-eclampsia, gestational diabetes and preterm birth. Earlier headlines claimed bigger benefits, and the update removed some studies that did not meet trustworthiness checks. The World Health Organization does not recommend vitamin D for all pregnant people solely to improve maternal and newborn outcomes. That position does not rule out treating deficiency or following your country's advice on nutritional adequacy.

The takeaway: aim for enough, and skip megadoses.

How much vitamin D do you need?

Authorities use different measures, so compare like with like.

Units can confuse. 1 mcg of vitamin D equals 40 IU, so 10 mcg is 400 IU, and 15 mcg is 600 IU. The US and EFSA figures are totals from food and pills, and the UK, Danish, Dutch and Irish figures are supplement doses, so they are not the same type of number.

Can you get enough from the sun?

Sunlight helps, and it is not a reliable plan in every place or season. How much vitamin D your skin makes depends on:

• Latitude and season. In northern countries the winter sun is too weak for skin to make much.
• Skin tone. Darker skin makes vitamin D more slowly.
• Clothing and time outdoors. Covering skin, working indoors or using sunscreen lowers production.
• Time of day and cloud cover.

For these reasons, sunlight is a poor basis for deciding how much you get. This does not mean you should skip sun safety, and a supplement can fill the gap safely.

Which foods contain vitamin D?

Food sources are limited, which is why supplements are common.

• Oily fish. Salmon, sardines, trout and herring. The omega-3 guide covers choosing lower-mercury species.
• Fortified foods. Check labels on milk, plant drinks, yogurt and cereals, because fortification differs by country.
• Eggs and some mushrooms. These add smaller amounts.

Most people do not reach their needs from food alone, particularly in winter. The best foods guide has more ideas for building balanced meals.

What about your prenatal vitamin?

A prenatal vitamin often contains vitamin D. Check the amount before adding a separate tablet, and add up the totals from everything you take. If your prenatal provides 10 mcg or 15 mcg, you may need nothing more. The prenatal vitamin guide shows how to read the label.

Who is more likely to be low?

Some groups carry a higher chance of low vitamin D:

• people with darker skin,
• people who cover most of their skin outdoors,
• people who spend most of the day indoors or live where winters are long,
• people with conditions that affect absorption, such as some bowel diseases or after weight-loss surgery,
• people with a higher body weight.

A blood test can be useful for these groups. If your result is low, your provider will choose a treatment regimen, and this is different from a routine daily supplement.

Should you take more to be safe?

More is not better. The adult upper limit is 100 mcg (4,000 IU) a day in both the US and EFSA's European assessment, and you should not need that much unless a provider has advised a treatment dose. Very high intake over time can raise blood calcium and cause harm. Check the totals from all your products. A multivitamin, a vitamin D tablet and fortified foods can add up.