Vitamin D, 25-Hydroxy

Vitamin D testing is an important aspect of prenatal care, as adequate levels of this nutrient are essential for maternal bone health, immune function, and fetal development. However, controversy surrounds vitamin D testing in pregnancy due to inconsistent guidelines for optimal levels, variations in testing methods, and differing opinions on the necessity of supplementation. This article covers recommended testing practices and common causes of both high and low levels.

Vitamin D Testing

The 25OH vitamin D (250H-D) is considered the most accurate test to assess vitamin D status.

When the skin is exposed to UVB radiation from sunlight, 7-dehydrocholesterol, a compound in the skin, absorbs the UVB rays. UVB radiation converts 7-dehydrocholesterol into previtamin D3. Previtamin D3 undergoes thermal isomerization (a change in structure induced by heat) in the skin, converting it into vitamin D3 (cholecalciferol). 

Dietary vitamin D, primarily in as vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol).

Vitamin D3 is then transported to tissues for conversion to calcidiol (the active hormonal form of vitamin D). This is primarily done in the kidneys, but other tissues, such as the placenta. 

Vitamin D in Pregnancy

During pregnancy, serum calcitriol rises from the first trimester, doubling by the end of the third trimester. This increase is due to an actual increase in production and not a reduction in renal clearance. This rise is driven by maternal kidney and placental production, with most conversion from maternal sources.

​While serum calcitriol levels rise, serum 25OH-D levels decrease due to increased demands.

Maternal need for dietary vitamin D increases in the breastfeeding period. A study found that a maternal intake of 6,400 IU of vitamin D was equivalent to supplementing 400 IU in the infant. This means that the higher consumption of vitamin D delivered the required 400 IU of vitamin D needed in breast milk to supply infant needs.

High Vitamin D in Pregnancy

Studies have shown no negative effects in pregnancy with serum levels <80 ng/mL. Greater than 100 ng/mL is associated with hyperkalemia risk.

Most Common Causes of High Vitamin D in Pregnancy

• Recently Taking Supplements
• Excess Supplementation
• Nutritional Deficiencies (magnesium, zinc)
• Poor Calcitriol Production
• Genetics

Low Vitamin D in Pregnancy

Studies have found that vitamin D deficiency increases the risk of preeclampsia, gestational diabetes, low birth weight, and preterm birth. Maternal vitamin D deficiency in pregnancy is also associated with long-term health risks for babies.

A 2023 study from Sweden found that serum vitamin D levels ≤30.1 nmol/L were associated with an increased risk of preeclampsia.11 Vitamin D status prior to 20 weeks is associated with gestational diabetes risk, with >30 ng/mL being protective.
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Studies have also found that a serum concentration of 40 ng/mL is needed to maintain calcitriol balance in pregnancy. To maintain these levels, a supplemental dosage of 4,000 IU per day was required.

Women living at higher altitudes, or above the 37th parallel, are more likely to have vitamin D deficiency. These individuals need to focus on increasing vitamin D-rich foods as well as supplements. Those with higher melanin levels living at higher altitudes and latitudes are at an even higher risk of vitamin D deficiency.

Most Common Causes of Low Vitamin D in Pregnancy

• Vitamin D Deficient Diet
• Inflammatory Bowel Diseases (Celiac Disease, Crohn's Disease, etc...)
• Low Fat Diet
• Low Sun Exposure
• Advanced Maternal Age

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Medical Disclaimer:
This content is for educational purposes only and is intended for licensed medical professionals. It is based on current research, subject to change, and not intended to diagnose, treat, or prevent any medical condition. This information does not replace consultation with a qualified healthcare provider. Seek professional medical evaluation and treatment from a licensed provider. Use of this information is at your own risk.​

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