Vitamin D and Fertility: How the “Sunshine Vitamin” Supports Conception, IVF Success, and Pregnancy Outcomes

A Hidden Factor in Fertility

Infertility is one of the most challenging health issues couples face, with millions worldwide affected. Assisted reproductive technologies such as in vitro fertilization (IVF) have offered hope, but success rates remain modest. As a result, researchers are exploring new ways to enhance outcomes, and nutrition has become an important focus.

One nutrient drawing increasing attention is vitamin D. Traditionally associated with bone health, vitamin D actually functions as a hormone with wide-reaching effects on the immune system, metabolism, and reproduction. Recent research suggests that vitamin D status may influence fertility in both men and women, affect IVF success, and even alter the course of pregnancy itself.

Vitamin D in Women: Fertility and IVF

Vitamin D receptors are found throughout the female reproductive system, including the ovaries, uterine lining, placenta, and even developing embryos. These receptors regulate key genes involved in implantation, immune tolerance, and hormone production. When vitamin D levels are insufficient, these delicate processes may be disrupted, reducing the chance of a successful pregnancy.

A large meta-analysis published in 2023 brought clarity to this question. Researchers pooled data from over 2,300 infertile women undergoing IVF or related procedures. The women who received vitamin D supplementation were 70 percent more likely to achieve a clinical pregnancy compared with those who did not. While vitamin D did not appear to reduce miscarriage risk or increase implantation rates, its impact on clinical pregnancy outcomes was significant.

Interestingly, dosage and duration mattered. The greatest improvements were seen in women who were deficient to begin with (vitamin D levels below 30 ng/mL) and who took daily supplements in the range of 1,000 to 10,000 IU for one to two months before embryo transfer. This suggests that steady, moderate supplementation is more effective than a one-time high dose.

Vitamin D in Men: Sperm Health and Fertility

Although fertility discussions often focus on women, male factors contribute to nearly half of infertility cases. Here too, vitamin D appears to play a role. Vitamin D receptors are found in the testes and in sperm-producing cells, influencing hormone synthesis and sperm function.

Men with higher vitamin D levels tend to have better sperm counts, improved motility, and healthier morphology. In fact, studies show that fertile men typically have higher average vitamin D levels compared to infertile men. There is also some evidence that vitamin D supports testosterone levels in older men, though this effect is less clear in younger populations.

Taken together, these findings suggest that vitamin D may not be a magic solution for male infertility, but it does help the reproductive system operate at its best. Optimizing vitamin D levels may therefore improve the overall quality of sperm and increase the chances of conception.

Vitamin D and PCOS: A Common Link

Polycystic ovary syndrome (PCOS) is one of the most common causes of infertility, affecting between 5 and 20 percent of women of reproductive age. The syndrome is characterized by hormonal imbalances, irregular cycles, excess androgens, and often insulin resistance. Strikingly, as many as 85 percent of women with PCOS are vitamin D deficient.

Vitamin D supplementation has been shown to improve several aspects of PCOS. Women with adequate vitamin D levels are more likely to ovulate regularly, and supplementation can help regulate menstrual cycles. In IVF settings, vitamin D deficiency is linked to poorer embryo quality, while supplementation improves outcomes, bringing pregnancy rates closer to those of women without PCOS. Vitamin D also reduces insulin resistance and lowers androgen levels, two of the underlying drivers of PCOS symptoms.

One striking finding is that ovulation probability rises with vitamin D status. In PCOS, the chance of ovulation is only 68 percent when vitamin D levels are below 20 ng/mL, but rises to nearly 80 percent once levels exceed 30 ng/mL. These small but meaningful improvements demonstrate how vitamin D status can influence fertility outcomes in this high-risk group.

Vitamin D and Pregnancy: Supporting Mother and Baby

Pregnancy places greater demands on the body, and vitamin D requirements rise accordingly. Despite this, around 70 percent of pregnant women are deficient, with only a small fraction achieving optimal levels. Deficiency during pregnancy carries risks for both mother and child.

Low vitamin D levels have been linked to gestational diabetes, preeclampsia, miscarriage, and preterm birth. Women with severe deficiency are also more likely to deliver by cesarean section, and babies may be more likely to have low birth weight. Beyond birth, maternal vitamin D deficiency may influence long-term outcomes for children, increasing risks for asthma, type 1 diabetes, and even certain neurodevelopmental disorders.

Adequate vitamin D, by contrast, supports healthy placental function, reduces inflammation, and promotes proper fetal skeletal development. It has also been associated with healthier birth weights and lower risks of infection-related complications such as bacterial vaginosis. Remarkably, breastfeeding women who take more than 4,000 IU of vitamin D daily can transfer enough of the nutrient through breast milk to meet their infants’ needs, highlighting how maternal status continues to influence child health beyond pregnancy.

Practical Guidance

The first step in optimizing vitamin D for reproductive health is testing. The best marker is serum 25-hydroxyvitamin D. For fertility and pregnancy support, most experts recommend aiming for levels of at least 30 ng/mL (75 nmol/L). Some evidence suggests that slightly higher levels, around 40 ng/mL, may provide additional protection against preterm birth.

For supplementation, daily intakes of 1,000 to 4,000 IU are generally effective and safe for most adults. In cases of deficiency, higher short-term doses may be prescribed under medical supervision. Because needs vary based on factors like skin tone, body weight, sun exposure, and geographic location, individualized guidance is best.

Toxicity is rare but possible with prolonged very high doses, usually above 10,000 IU daily for months. Symptoms include nausea, weakness, and elevated calcium levels. For this reason, supplementation should always be monitored by a healthcare professional, especially in pregnancy.

Looking Ahead

Although the evidence is strong, important questions remain. Researchers are still determining the optimal vitamin D levels specifically for reproductive outcomes, the best timing of supplementation, and whether genetic factors explain why some individuals respond better than others. More large-scale clinical trials are needed, but what we know already is promising.

A Simple Step with Big Impact

Vitamin D is inexpensive, widely available, and safe when used appropriately. Research shows it can improve fertility in both men and women, enhance IVF outcomes, support women with PCOS, and reduce pregnancy complications. It is not a cure-all, but optimizing vitamin D may be one of the simplest yet most powerful steps couples can take on their fertility journey.

If you are planning pregnancy, undergoing fertility treatment, or managing PCOS, talk to your healthcare provider about testing your vitamin D status. The sunshine vitamin might be a missing piece in your path to parenthood.