Most fertility specialists recommend 1,000 to 4,000 IU of vitamin D daily when trying to conceive, with the goal of reaching a blood level of at least 30 to 40 ng/mL. The exact dose you need depends on where your levels are now, which is why getting a simple blood test before you start supplementing is the smartest first step.
Why Vitamin D Matters for Conception
Vitamin D receptors are found throughout the reproductive system, in the ovaries, uterus, placenta, and pituitary gland. This isn’t a vitamin that just supports bone health. In lab studies, treating ovarian cells with vitamin D increased production of progesterone and estrogen, two hormones essential for ovulation and maintaining early pregnancy. In animal models, removing the vitamin D receptor entirely led to impaired follicle development and reduced estrogen production.
Vitamin D also plays a direct role in preparing the uterine lining for implantation. It promotes the growth and transformation of endometrial cells in a dose-dependent way, meaning higher vitamin D levels (up to a point) lead to a thicker, more receptive lining. One study of infertile women in China found that 28.3% were vitamin D deficient, and that deficiency was linked to significantly lower clinical pregnancy and live birth rates during IVF, even after controlling for embryo quality. Researchers believe this points to vitamin D’s specific effect on the uterine environment rather than egg quality alone.
The Blood Levels to Aim For
A blood level below 20 ng/mL is considered deficient. Between 20 and 30 ng/mL is insufficient. For general health, the optimal range is 20 to 50 ng/mL, but fertility-focused guidance sets the bar higher: at least 40 ng/mL is recommended from the start of pregnancy, so ideally you’d reach that level before conceiving.
The difference these numbers make is striking. In a study of 173 women undergoing IVF, those with sufficient vitamin D levels had a clinical pregnancy rate of 52.5% per cycle, compared to just 34.7% for women with insufficient or deficient levels. Pregnancy rates per embryo transfer showed a similar gap: 54.7% versus 37.9%.
How Much to Take Daily
Your ideal dose depends on your starting level. If you’re already in the 20 to 30 ng/mL range, a daily supplement of 1,000 to 2,000 IU will typically bring you into the target zone within about three months. If you’re below 20 ng/mL (true deficiency), you’ll likely need 3,000 to 4,000 IU daily, and it can take six months or longer to reach 30+ ng/mL.
A longitudinal study tracking monthly supplementation found that people starting in the deficient range reached an average of about 23 ng/mL after three months and 33 ng/mL after 12 months on a moderate dose (equivalent to roughly 1,700 IU daily). Those who started in the insufficient range crossed the 30 ng/mL threshold within three months on the same regimen. The takeaway: if you’re significantly deficient, start supplementing well before you want to conceive, ideally three to six months ahead.
The European Food Safety Authority sets the tolerable upper intake level at 4,000 IU (100 micrograms) per day for adults, including women who are pregnant or breastfeeding. Staying at or below this level is considered safe without medical supervision. Some practitioners prescribe higher loading doses for severe deficiency, but that’s a clinical decision based on bloodwork.
Vitamin D3 vs. D2
Choose vitamin D3 (cholecalciferol) over D2 (ergocalciferol). D3 is the form your skin makes from sunlight, and it raises blood levels more effectively and sustains them longer. Most over-the-counter supplements are D3. Taking it with a meal that contains some fat improves absorption since vitamin D is fat-soluble.
Your Partner’s Vitamin D Levels Matter Too
Vitamin D receptors are also found in the testes, and deficiency in men is linked to reduced sperm quality. A meta-analysis of randomized trials found that vitamin D supplementation significantly improved both total sperm motility and progressive motility (the percentage of sperm swimming forward effectively) compared to placebo. It did not significantly increase sperm count or concentration, but motility is one of the most important factors in whether sperm can reach and fertilize an egg.
The doses used in these trials ranged from 1,400 IU to 4,000 IU daily over periods of roughly 10 to 22 weeks. For couples trying to conceive, it’s worth having both partners checked and supplementing together if levels are low.
Effects on Ovarian Reserve
Anti-Müllerian hormone (AMH) is a key marker of ovarian reserve, reflecting how many eggs remain. The relationship between vitamin D and AMH depends on whether you have PCOS. In women who ovulate normally, vitamin D supplementation significantly increased AMH levels, a positive sign for egg supply. In women with PCOS, supplementation actually decreased AMH, but this is also considered beneficial because abnormally high AMH in PCOS reflects disordered follicle development rather than a large healthy egg reserve. Bringing it down suggests the ovaries are functioning more normally.
When to Get Tested and What to Expect
The test you need is a 25-hydroxyvitamin D blood test, sometimes written as 25(OH)D. It’s inexpensive and widely available. If you’re planning to conceive in the next few months, get tested now so you know whether you need a standard maintenance dose or a higher corrective dose.
Plan to retest after three months of consistent supplementation to see where your levels have landed. If you started below 20 ng/mL, you may need a second recheck at six months. The goal is to enter pregnancy with a level of at least 40 ng/mL, since vitamin D demands increase during pregnancy and your stores will be drawn down as the fetus develops.