The Importance of Vitamin D and Fertility
Something I get asked about all the time is vitamin D. I know supplements can feel overwhelming, especially when it comes to what you really need, and when. And let’s be honest, nutrition research is tough—it’s hard to design these studies, but that doesn’t mean the evidence we do have isn’t important. So, let’s break it down together.
What Is Vitamin D?
Vitamin D is a fat-soluble vitamin, meaning it gets stored in your fat tissues—unlike water-soluble vitamins (like B vitamins), which your body just pees out if you get too much. Because vitamin D can build up, you can technically get too much, which is why we don’t just tell everyone to take massive doses all the time.
That said, vitamin D deficiency is actually pretty common, with up to 40% of people being deficient. Toxicity is much rarer and usually happens when someone takes really high doses daily without being monitored.
Symptoms: Too Much vs. Too Little
If you do get too much vitamin D, symptoms might include:
Nausea
Weight loss
Irregular heartbeat
Calcium deposits (think: kidney stones)
More commonly, though, people are deficient, which can show up as:
Fatigue
Increased risk of osteoporosis or fractures
Subtle or no symptoms at all
Who’s More Likely to Be Deficient?
A lot of things can contribute to low vitamin D levels:
Malabsorption (Crohn’s, ulcerative colitis, or GI surgeries)
Low dietary intake (especially if you're vegan, like me)
Lack of sun exposure
Obesity (more fat = more vitamin D stored in fat = less available in circulation)
Darker skin (which absorbs less UVB light needed to make vitamin D)
So your skin tone, your lifestyle, your health, your diet, and even where you live can impact your vitamin D status.
Why Vitamin D Matters in Pregnancy
When you’re pregnant (or trying to conceive), vitamin D becomes even more important. It helps your body absorb calcium and plays a key role in bone development for your baby. That’s why every prenatal vitamin contains vitamin D—usually around 600–800 IU.
You should ideally be taking a prenatal at least three months before trying to conceive—not just for the folic acid and neural tube development, but for vitamin D and bone health, too.
Where Do We Get Vitamin D?
There are four main sources:
Food: Fatty fish, eggs, liver
Fortified foods: Milk, cereals, plant-based milks (these have added vitamin D)
Supplements: Most commonly D3, which is made from animal sources (usually lanolin from sheep’s wool). D2 is vegan-friendly but not absorbed quite as well.
Sunlight: Your skin can make vitamin D when exposed to UVB rays. But darker skin, sunscreen, and spending more time indoors can reduce this.
Fertility and Vitamin D: What Does the Research Say?
This is where it gets tricky. The studies are all over the place—some use different units (nanograms vs. nanomoles), and different thresholds (20 vs. 30 ng/mL), making comparisons a little messy. But here’s what we know so far:
Natural Conception:
In the general population, we’re not seeing major differences in natural pregnancy rates between people who are vitamin D deficient and those who aren’t.
That said, certain subgroups (like those with underlying health issues or in certain regions) may benefit more.
IVF & Embryo Transfer:
Some IVF studies show better live birth rates in people with normal vitamin D levels.
Vitamin D might impact implantation, not necessarily egg quality.
In egg donor cycles (where the egg quality is controlled), low vitamin D in recipients has been linked to lower pregnancy rates.
So, many fertility clinics aim for vitamin D levels over 30 ng/mL, especially before an embryo transfer. If your level is under 20, some clinics may even postpone transfer until you’ve repleted your vitamin D.
What About PCOS, Endometriosis, or Male Fertility?
PCOS:
No clear improvements in metabolic markers with supplementation.
But in some IVF patients with PCOS, higher pregnancy rates were seen when vitamin D was repleted.
AMH & Egg Reserve:
Not consistent data showing a direct effect on ovarian reserve (AMH).
Some studies did note a drop in AMH with vitamin D supplementation in PCOS, which might signal more normal ovulation patterns—but this is still under study.
Endometriosis:
Hard to study because diagnosis requires surgery.
No strong data yet, but some possible benefit to implantation rates.
Male Fertility:
Mixed results. Some improvement in sperm parameters, some not. Still unclear.
Should You Supplement?
Here’s my general advice:
Take a prenatal every day (starting 3 months before TTC).
If your vitamin D is low, add 1,000–2,000 IU of D3 daily.
Don’t exceed 4,000–5,000 IU/day without doctor supervision.
If you’re vegan, go for D2, but know that D3 raises your levels more effectively.
Vitamin D isn’t magic—but it’s a small, important piece of the fertility puzzle, especially when it comes to implantation and pregnancy. It’s easy to supplement, relatively safe at appropriate doses, and super important for your overall health.
I recommend checking your vitamin D level and working with your doctor to get it to a goal of 30 ng/mL if you’re trying to conceive or going through fertility treatment. And remember—your prenatal is your foundation, but sometimes we need a little extra boost.
