Most prenatal vitamins contain 400 to 600 IU of vitamin D. A growing body of research suggests that isn't nearly enough — and a landmark 2024 study following children for six years shows why the amount you take in pregnancy matters far longer than anyone assumed.
Here's what the science actually says about vitamin D in pregnancy, how much you need, and why the form and delivery matter as much as the dose.
How much vitamin D should you take during pregnancy?
The standard recommendation is 600 IU per day. Most research on pregnancy outcomes suggests that number is too low to reach vitamin D sufficiency in the majority of pregnant women.
Clinical trials have used 1,000 to 4,000 IU per day, and studies have found 4,000 IU daily to be both safe in pregnancy and substantially more effective at raising and maintaining vitamin D levels than the standard dose. The National Institutes of Health and multiple obstetric researchers have pointed to the gap between the official recommendation and what the evidence supports.
In short: 600 IU is the floor, not the target. The doses that produce measurable benefits in clinical trials are considerably higher.
What the MAVIDOS study found about vitamin D and children's bones
The most compelling recent evidence comes from the Maternal Vitamin D Osteoporosis Study (MAVIDOS), a randomized, placebo-controlled trial — the strongest form of evidence in nutrition science.
Researchers recruited more than 1,000 pregnant women across three UK hospitals. From roughly 14 weeks of pregnancy until delivery, each woman took either 1,000 IU of vitamin D3 (cholecalciferol) per day or a placebo. Then the researchers did something most studies never do: they followed the children for years afterward.
A 2024 paper published in the American Journal of Clinical Nutrition reported the results of DXA bone scans on these children at ages six and seven. The finding: children whose mothers took vitamin D during pregnancy had greater bone mineral density than children whose mothers took the placebo.
This effect was not visible at birth. It emerged and persisted into mid-childhood. A nutritional choice made during pregnancy was still measurable in a child's skeleton nearly a decade later.
The lead researcher framed it as a public-health strategy: stronger childhood bones, and a potentially lower risk of fractures and osteoporosis later in life — all traced back to vitamin D status during pregnancy.
Why does vitamin D in pregnancy affect a child's bones years later?
Vitamin D regulates how the body absorbs and uses calcium and phosphate — the minerals bones are built from. During pregnancy, the developing fetus draws on the mother's vitamin D and calcium to build its skeleton.
If the mother's vitamin D status is low, the raw materials for building strong bone are in shorter supply during a critical developmental window. The MAVIDOS follow-up suggests that adequate vitamin D during this window sets a higher baseline for bone density that the child carries forward for years.
Pregnancy isn't only about reaching delivery day in good health. It's laying down foundations the child keeps. Vitamin D is one of the clearest, best-documented examples of that principle.
Are most pregnant women deficient in vitamin D?
Vitamin D insufficiency is widespread. Depending on the threshold used, a large share of adults — and pregnant women specifically — fall below the level considered sufficient. The problem worsens in winter, when less sunlight reaches the skin, and vitamin D levels tend to drop further during pregnancy itself as the body's demands increase.
Despite this, most prenatal care does not routinely test vitamin D levels, and many providers don't test even when asked. If you're pregnant or trying to conceive, the lab to request is 25-hydroxyvitamin D. It's inexpensive, and it tells you your actual status rather than leaving you to guess. A level below 30 ng/mL is generally considered insufficient and worth a conversation about supplementation.
Does the form of vitamin D matter? D3 vs D2
Yes. Vitamin D comes in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). D3 is the form your body produces from sunlight and is generally more effective at raising and maintaining blood levels of vitamin D. The MAVIDOS trial used D3, and it's the form most researchers favor for supplementation.
When choosing a prenatal, D3 is the form to look for.
Why how you take vitamin D matters as much as how much
Here's a detail that gets almost no attention: vitamin D is fat-soluble. Your body absorbs it best when it arrives alongside dietary fat.
That creates a problem with how most prenatals are actually taken — swallowed as a pill first thing in the morning, often with nothing but black coffee. Taking a fat-soluble vitamin on an empty stomach is close to the worst-case scenario for absorbing it. The nutrient may be on the label, but that doesn't guarantee your body can use it.
This is where the delivery format of a prenatal genuinely matters. Vitamin D — along with the other fat-soluble vitamins, A, E, and K — absorbs meaningfully better when consumed with food that contains fat. A prenatal eaten as part of a food matrix that includes fat gives these nutrients a real chance to be absorbed, rather than passing through.
What this means for choosing a prenatal
Putting the evidence together, three things matter for vitamin D in a prenatal:
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Dose. Look for a meaningful amount — well above the 600 IU standard. The clinical evidence supports doses in the 1,000 to 4,000 IU range, with 4,000 IU shown to be safe and effective in pregnancy.
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Form. D3 (cholecalciferol), not D2.
- Delivery. A format that includes dietary fat, so the vitamin D is actually absorbed rather than wasted.
Most prenatals get one of these right, at best. Many contain 400 to 600 IU of vitamin D in a pill designed to be swallowed on an empty stomach — a low dose in a format that undercuts absorption.
How Tend approaches vitamin D
Tend was built around exactly these principles. Each serving delivers 4,000 IU of vitamin D3 — the dose the clinical research supports, and more than six times the standard recommendation.
Just as importantly, it's delivered inside a soft-baked bar that contains dietary fat. The vitamin D3, along with the other fat-soluble vitamins, is eaten with food that helps your body absorb it — not swallowed dry on an empty stomach. The right nutrient, at the right dose, in a form your body can actually use.
That combination — dose, form, and delivery — is the difference between a number on a label and a nutrient your body absorbs.
Frequently asked questions
Is 4,000 IU of vitamin D safe during pregnancy?
Multiple clinical trials have studied 4,000 IU of vitamin D per day in pregnancy and found it both safe and more effective than lower doses at achieving vitamin D sufficiency. As with any supplement in pregnancy, discuss your specific needs with your provider, ideally informed by a 25-hydroxyvitamin D test.
When should I start taking vitamin D in pregnancy?
The MAVIDOS trial began supplementation at around 14 weeks. Many experts recommend ensuring adequate vitamin D from before conception through pregnancy and breastfeeding, since the demands are continuous.
Can I get enough vitamin D from sunlight alone?
For most pregnant women, especially in winter, at higher latitudes, or with limited sun exposure, sunlight alone is not a reliable source. This is why testing and supplementation are commonly recommended.
What vitamin D level should I aim for in pregnancy?
A 25-hydroxyvitamin D level of at least 30 ng/mL is generally considered sufficient, though some researchers argue for higher targets. Testing is the only way to know your status.
Does vitamin D in pregnancy help the baby or the mother?
Both. Adequate vitamin D supports maternal health and, as the MAVIDOS follow-up shows, has measurable, lasting effects on the child's bone development years after birth.
This article is for educational purposes and isn't a substitute for medical advice. Talk to your healthcare provider about your individual needs, ideally with a 25-hydroxyvitamin D test to guide the conversation.
Sources: Moon RJ, et al. Pregnancy vitamin D supplementation and offspring bone mineral density in childhood follow-up of a randomized controlled trial. American Journal of Clinical Nutrition, 2024. · Curtis EM, et al. MAVIDOS, JBMR Plus, 2022.
