The short answer: start as early as possible, ideally before you conceive. Vitamin D plays a role in fertility, placental health, and fetal bone development from the earliest weeks of pregnancy, and roughly 68% of pregnant women already have insufficient levels by the end of their first trimester. If you’re not yet pregnant but planning to be, building up your vitamin D stores now gives you a head start. If you’re already pregnant, starting today is better than waiting.
Before Conception Is the Ideal Window
Your vitamin D status at the time of conception matters more than many people realize. In a study of women undergoing fertility treatment, those with sufficient vitamin D levels before conception were significantly more likely to achieve a successful pregnancy. The difference was measurable: women with blood levels at or above 50 nmol/L had roughly twice the odds of a positive outcome compared to those below that threshold.
This makes sense biologically. Vitamin D helps regulate calcium absorption and supports immune function, both of which are involved in implantation and early placental development. If you’re actively trying to conceive, a daily supplement is a simple way to make sure your levels are adequate before pregnancy even begins.
First Trimester and Beyond
If you didn’t start supplementing before pregnancy, your first prenatal visit is a natural time to begin. Many prenatal vitamins include some vitamin D, but the amount varies widely, and it may not be enough on its own. The NHS recommends that all pregnant women take 10 micrograms (400 IU) of vitamin D daily. The Institute of Medicine suggests 400 to 600 IU per day as a baseline.
Deficiency tends to worsen as pregnancy progresses. A large meta-analysis found that 68% of women had low vitamin D in the first trimester, climbing to 81% by the second trimester before dropping slightly to 70% in the third. This pattern likely reflects the growing fetus drawing more heavily on maternal stores as it develops. The takeaway: the earlier you start supplementing, the less likely you are to fall behind.
Why Vitamin D Matters During Pregnancy
The most well-documented benefit is a reduction in preeclampsia risk. A systematic review and meta-analysis covering multiple trials found that vitamin D supplementation reduced preeclampsia risk by about 45%. That’s a substantial effect for a single, inexpensive supplement. The benefit was strongest when women who previously received no vitamin D began supplementing, compared to women who simply increased an already existing dose. One screening-based study found even larger reductions: 60% lower preeclampsia risk, 50% lower gestational diabetes risk, and 40% fewer preterm deliveries.
Vitamin D also supports fetal bone development by helping your body absorb and regulate calcium. Your baby’s skeleton begins forming early and demands a steady supply of calcium throughout pregnancy. While the fetus has some protective mechanisms that buffer it from moderate maternal deficiency, maintaining adequate levels gives both you and your baby the best foundation.
How Much to Take
Standard recommendations range from 400 to 600 IU per day during pregnancy. This is the amount found in many prenatal vitamins and reflects guidelines from the Institute of Medicine. Some practitioners recommend higher doses for women who are already deficient, but the tolerable upper limit for pregnant women is 4,000 IU per day. Staying below that ceiling is considered safe for both mother and baby, and no separate upper limit exists for pregnancy compared to the general adult population.
If you have darker skin, spend most of your time indoors, live at a northern latitude, or consistently wear clothing that covers most of your body, you’re at higher risk for deficiency year-round. The NHS specifically notes that women in these categories should consider daily supplementation throughout the entire year, not just during the winter months when sunlight is limited. For everyone else, supplementation between September and March (when your body can’t produce enough vitamin D from sunlight alone) is the minimum recommendation.
Which Form of Vitamin D to Choose
Most over-the-counter supplements contain vitamin D3, which is the form your skin naturally produces in response to sunlight. Vitamin D2, the plant-derived form, is also available but is generally considered less effective at raising blood levels. Both are widely used and safe during pregnancy.
A newer option is calcifediol, a pre-converted form of vitamin D that your body can use more readily. In animal studies, calcifediol doubled vitamin D levels in both maternal and fetal blood compared to the same amount of standard vitamin D3. It also raised blood levels more quickly. Human studies in pregnant women are still limited, but the early evidence suggests it could be a more efficient option, particularly for women who struggle to raise their levels with standard supplements.
Factors That Increase Your Risk of Deficiency
- Skin tone: Melanin reduces the skin’s ability to produce vitamin D from sunlight, so women with darker skin need more sun exposure or a higher supplement dose to maintain the same levels.
- Geography: Living above roughly 37 degrees latitude (think San Francisco, Athens, or Seoul) means little to no vitamin D production from sunlight during winter months.
- Diet: Vegetarian and vegan diets tend to be lower in vitamin D since the richest food sources are fatty fish, egg yolks, and fortified dairy products.
- Body weight: Vitamin D is fat-soluble, so higher body fat can sequester more of it away from circulation, effectively lowering the amount available for use.
- Sunscreen and clothing: Consistent use of sunscreen or full-coverage clothing blocks the UV rays needed for vitamin D synthesis.
If any of these apply to you, a blood test measuring your vitamin D level (called a 25-hydroxyvitamin D test) can tell you exactly where you stand. A level at or above 50 nmol/L (20 ng/mL) is generally considered sufficient, though some experts argue the threshold should be higher during pregnancy. Knowing your number lets you and your provider choose the right dose rather than guessing.