Vitamin D: Why is it so Important for Fertility & Pregnancy?

By Chloe Dymond Young - BANT Registered Nutritional Therapist, IBCLC and ex-Midwife (17 years).

Why Vitamin D Matters for Fertility, IVF & Pregnancy:

Vitamin D is often associated with bone health, but its role in reproductive health, immune regulation, and foetal development is equally critical. In the context of trying to conceive naturally or through IVF, or already being pregnant, vitamin D status can be a powerful and under appreciated factor influencing success and outcomes.

Let’s explore what the research says about vitamin D, the risks of deficiency, how it impacts both parents and baby, and what you can do to optimise your levels - especially in the UK, where deficiency is common.

Prevalence of Deficiency in the UK:

In the UK, vitamin D deficiency is widespread due to limited sunlight, particularly from October to March. According to Public Health England, over 1 in 5 adults have low vitamin D levels, with higher rates among:

• People with darker skin (African, South Asian, Caribbean backgrounds)

• Those who cover most of their skin or stay indoors

• Pregnant women and those trying to conceive

Deficiency is defined as a blood 25(OH)D level of <30 nmol/L, but many experts argue that optimal levels for fertility and pregnancy are >75 nmol/L.

Why Vitamin D Matters for Fertility & IVF:

Vitamin D plays a role in:

• Ovarian function and egg maturation

• Endometrial receptivity (implantation potential)

• Hormone regulation, including AMH and progesterone

• Reducing autoimmunity, which is implicated in some unexplained fertility cases

A 2020 systematic review and meta-analysis in Human Reproduction found that women with sufficient vitamin D levels had significantly higher pregnancy and live birth rates during IVF cycles.

In men, vitamin D is linked to:

• Better sperm motility

• Increased testosterone production

Vitamin D and Pregnancy Outcomes:

Low maternal vitamin D levels are associated with:

• Increased risk of miscarriage

• Pre-eclampsia

• Gestational diabetes

• Low birth weight and preterm birth

• Impaired skeletal development and immune function in the baby

The first 1,000 days (from conception to age two) are critical for lifelong health. Vitamin D helps regulate gene expression involved in brain development, bone growth, and immune tolerance.

Research from The Journal of Clinical Endocrinology & Metabolism has shown that vitamin D status in pregnancy can influence the child's risk of asthma, allergies, and autoimmune diseases.

Genetic SNPs That Affect Vitamin D Levels:

Certain genetic variants can make you more prone to deficiency, even with moderate sun exposure or supplementation:

• GC gene (Vitamin D binding protein) – affects transport and availability of vitamin D

• CYP2R1 – influences the liver's ability to convert vitamin D to its active form

• VDR gene – affects how vitamin D interacts with cells and DNA

Understanding your genetic predisposition can explain why some individuals need higher doses or different forms (e.g., D3 with K2) to maintain optimal levels.

Other Causes of Low Vitamin D:

Beyond sunlight and genetics, other health issues can deplete or impair vitamin D metabolism:

1. Chronic Inflammation

Inflammation increases the breakdown of vitamin D and reduces its receptor sensitivity.

2. Poor Gut Health & Malabsorption

Conditions like IBD, celiac disease, SIBO, and leaky gut can impair absorption of fat-soluble vitamins like D.

3. Autoimmune Conditions (AID)

Vitamin D helps modulate the immune system. Low levels are commonly found in those with autoimmune disorders such as Hashimoto’s, lupus, and rheumatoid arthritis - all of which can affect fertility.

4. Endometriosis

Women with endometriosis often have lower circulating vitamin D and reduced immune tolerance, which may affect implantation and increase inflammatory cytokines.

A 2022 review in Reproductive Sciences linked low vitamin D levels with increased endometriosis severity and reduced IVF success in affected women.

How to Optimise Vitamin D Levels:

1. Sunlight

• Aim for 15–30 minutes of sun exposure on face and arms in spring/summer, without sunscreen.

• Darker skin tones require longer exposure.

• UVB rays aren’t strong enough in UK winters, so supplementation is usually needed.

2. Food Sources (limited but helpful)

• Egg yolks

• Wild-caught oily fish (e.g., salmon, sardines)

• Mushrooms exposed to UV light (especially for vegetarians/vegans)

Note: Diet alone is usually insufficient to meet optimal levels.

3. Supplementation

• D3 is more effective than D2; choose vegan D3 if needed.

• Test 25(OH)D levels before and during pregnancy

Finally:

Vitamin D is one of the most foundational nutrients for reproductive health, yet it’s also one of the most commonly overlooked. Whether you’re trying to conceive, undergoing IVF, or already pregnant, checking and optimising your vitamin D status is a simple, powerful way to support both your fertility and the lifelong health of your baby.

If you’d like support optimising your nutrition status pre-conception, I offer one-off appointments at my Women and Baby clinic where I can help you navigate blood tests and supplements for a healthy pregnancy. Book via my services page.