Should I take vitamin D when trying to conceive?

The short answer: don't be low, especially early

Vitamin D helps a baby build bones, and researchers think it plays a part in brain development too. In the US, the recommended amount in pregnancy is 600 IU (15 mcg) a day, the same as before pregnancy, with a safe upper limit of 4,000 IU a day. The UK's NHS recommends a 400 IU (10 mcg) supplement in pregnancy.

Many prenatals include vitamin D, so check your label before adding anything. The goal is to start pregnancy with a normal level, because the research points to early pregnancy as the window that matters most, and that window opens before most people know they're pregnant.

What the studies found

Two US cohort studies hint that timing matters:

• Memphis (the CANDLE study, 1,019 mothers). 45.6% had vitamin D below 20 ng/mL in mid-pregnancy. Higher levels were linked to slightly higher IQ scores in their children at ages 4 to 6, about 1 point for every 10 ng/mL.
• The national ECHO program (912 mother-child pairs). A similar small link showed up with thinking scores at ages 7 to 12. The biggest gap between children with the highest and lowest scores showed up in their mothers' vitamin D levels in early pregnancy.

These are observational studies. They show links, not proof, and the differences are small.

Now the balance. In a Danish trial, COPSAC2010, 623 pregnant women took either 2,800 IU or 400 IU a day. When their children were followed to age 6, there was no difference in milestones, thinking, language or behavior.

Put together, the picture fits “don't be low, especially early” much better than “take a lot.” That's a reason to ask about your vitamin D now, when there's time to fix a low level before the early weeks.

Free: the 60-second Prenatal Label Check

Check your prenatal vitamin against what the research says a growing brain needs, and get Part 1 of the book free.

First name Email

Send me the free Label Check

By requesting the check you agree to receive emails from Grow a Genius. No spam; unsubscribe in one click. Privacy policy.

Who is more likely to run low

According to the NIH, these groups are more likely to have low vitamin D:

• People with darker skin
• People who get little sun
• People with obesity

One handy fact: the kind of sunlight that makes vitamin D doesn't pass through glass, so a sunny window seat won't help. Long winters, office jobs and covered skin all add up.

If any of these fit you, ask your doctor or midwife whether a vitamin D test makes sense, or whether they'd simply suggest a supplement. It belongs on your preconception list alongside thyroid, iron and blood sugar (see what blood tests to get before trying).

Testing: what to ask and what the numbers mean

A vitamin D blood test measures a form called 25-hydroxyvitamin D. Your doctor will read your result against the lab's range and your health. For a sense of how common low levels are: in the Memphis study above, nearly half of pregnant women (45.6%) were below 20 ng/mL in mid-pregnancy.

Useful questions for your preconception visit:

• “Given my skin tone, sun exposure and weight, should I be tested, or just take a supplement?”
• “My prenatal has [X] IU. Is that enough for me?”
• “If I'm low, how much should I take, and when should we recheck?”

The value of asking now is time. A low level found three months before conception can be back in range by the early weeks. A low level found at the first prenatal visit has already missed part of the window the ECHO data point to.

Food sources and how to get enough

Few foods are rich in vitamin D, but some help a lot:

Figures from the NIH Office of Dietary Supplements.

Salmon and trout pull double duty: they're also among the best sources of DHA, the omega-3 fat that builds the brain, and both are on the low-mercury list for pregnancy (see which fish are safe in pregnancy). Eggs bring choline as well.

For most people, a prenatal with vitamin D plus a few of these foods covers it. If your test comes back low, your doctor can tell you how much to take and for how long.

Why more isn't better

It's easy to read “low vitamin D linked to lower scores” and think high doses must be better. The Danish trial says otherwise: seven times the standard dose didn't change children's development by age 6.

Stay under the 4,000 IU daily upper limit unless your doctor has prescribed more for a measured deficiency. Count everything: your prenatal, any separate vitamin D, and fortified drinks or powders.

Vitamin D is one piece of the early-weeks plan. Folic acid, iodine and iron matter at the same time. See how to prepare your body for pregnancy for the full 90-day list.

Looking ahead: vitamin D after the birth

Vitamin D doesn't stop mattering at birth. The AAP recommends that all infants get at least 400 IU of vitamin D a day, starting soon after birth. The CDC says breastfed babies, including those who get some formula, need a 400 IU supplement every day. Formula has vitamin D added, but the CDC says a supplement is only unnecessary once a baby drinks 32 oz (about 950 mL) or more of formula a day, and many young babies drink less. Use drops made for infants, at exactly the dose on the label. More in vitamin D drops for babies.

Go deeper

Grow a Genius

In the book: Chapter 4: Why the Clock Starts Before the Test Turns Positive

Grow a Genius puts vitamin D into a 90-day countdown with iodine and folate, and its check-up chapter tells you exactly which tests to ask for and why.

• The 7 Genius Builders, stage by stage
• 16 printable guides, including the Prenatal Label Decoder
• Built on 500+ sources from 35+ countries
• 90-day Double Guarantee

See the plan