In pregnancy, Vitamin D plays a crucial role in the course of pregnancy, calcium supply, fetal bone mass development, and the regulation of the mother’s immune system. Maternal serum 25-hydroxycholecalciferol (25(OH)D) concentrations are the most reliable indicator of the body’s Vitamin D status.
25(OH)D is a blood marker for Vitamin D – this is a value from your blood that tells us how much Vitamin D your body has stored.
Vitamin D is a hormone-like vitamin that is crucial for:
strong bones and teeth
a healthy immune system
energy and well-being
muscle function
25(OH)D is the most reliable way to measure whether you have a Vitamin D deficiency or sufficient levels.
(professionally: 25-hydroxyvitamin D = 25(OH)D, the storage form of Vitamin D in the blood)
Why is this important?
Because the sun doesn’t always provide enough Vitamin D, even if we spend a lot of time outdoors, and we can feel tired, frequently ill, have problems with bones, muscles, or a weakened immune system, without knowing that the reason is low Vitamin D.
Measuring Vitamin D in pregnancy is not always entirely reliable. A blood test usually shows the total amount of Vitamin D, but not necessarily the part that the body can actually use.
During pregnancy, hormonal changes increase the protein that binds Vitamin D. Therefore, the lab result might appear normal, even though the body actually has insufficient available Vitamin D.
This is another reason why regular Vitamin D supplementation is often recommended during pregnancy, regardless of a single measurement.
Physiology and Placental Transfer
During pregnancy, serum 25(OH)D levels change little and primarily increase with greater sun exposure or higher intake. 25(OH)D crosses the placenta and determines fetal Vitamin D levels. Concentrations in umbilical cord blood are 10–40% lower than in maternal serum. The active form of Vitamin D, 1,25(OH)2D, does not cross the placenta but is synthesized within the placenta.
Doubled 1,25(OH)2D values in maternal blood enable a 2-fold higher absorption of the Vitamin D substrate 25(OH)D, which improves calcium absorption from the intestine. This creates conditions for normal fetal bone mass development. Despite a 100% increase in 1,25(OH)2D, maternal serum calcium does not rise, as it is excreted via urine. Vitamin D is also crucial in preventing the mother’s immune response to fetal antigens.
It affects innate immunity by stimulating the production of antimicrobial peptides, such as cathelicidin and β-defensin, and acquired immunity by acting on T lymphocytes.
Newborns and pregnant women belong to the at-risk group for Vitamin D deficiency.
Research shows that Vitamin D deficiency is common in premature infants, especially those with very low birth weight. This is closely linked to low Vitamin D levels in pregnant women.
Premature infants generally have lower Vitamin D levels than full-term newborns. While low Vitamin D levels do not increase mortality, they are associated with a more severe course of infections and longer hospitalization. Low Vitamin D increases the risk of eczema, atopic dermatitis, and asthma because Vitamin D regulates immune system function.
Maternal and newborn Vitamin D deficiency can also lead to low calcium in the child, which may manifest a few days after birth and, in severe cases, cause convulsions (neurogenic seizures).
Vitamin D also plays an important role in the development and mineralization of fetal and newborn bones.
Vitamin D Levels in Slovenian Pregnant Women
Two important studies were conducted in Slovenia:
📍 Maribor (2013/2014, 398 pregnant women, measured at birth):
Average concentration: 43.4 nmol/L
Deficiency <50 nmol/L: 65.1% of pregnant women
Severe deficiency <25 nmol/L: 23.6%
Optimal level ≥75 nmol/L: 9.5%
Umbilical cord blood values showed severe Vitamin D deficiency in 18% of newborns. This was associated with an increased risk of:
prematurity,
neonatal respiratory distress syndrome,
hospitalizations in the first year due to acute gastroenterocolitis or acute respiratory infections.
📍 Ljubljana (2011/2012, 132 pregnant women, third trimester):
Average concentration: 73.2 nmol/L
Deficiency <50 nmol/L: 14%
Insufficient level 50–75 nmol/L: 41%
Optimal level ≥75 nmol/L: in <50%
Only 2 women took Vitamin D or fish oil.
🔎 Seasonal influences:
→ The risk of deficiency is 2.3 times higher from October to March and 3.7 times lower if the pregnant woman exercised outdoors at least twice a week.
🌍 Global deficiency:
56% of pregnant women worldwide
74% of newborns
Vitamin D Deficiency and Pregnancy Complications
Research is not uniform, as many factors influence complications: body mass, genetics, lifestyle, and weight gain during pregnancy.
Gestational diabetes: can be a cofactor in disease development.
Preeclampsia: meta-analyses show a reduced risk with supplementation, especially when combined with calcium.
Hypertensive disorders in pregnancy: the risk is higher when 25(OH)D is <50 nmol/L.
Preterm birth: American studies show a 62% lower risk when 25(OH)D is >100 nmol/L at birth compared to <50 nmol/L.
Randomized studies have not confirmed an impact on the risk of C-section.
Recommended Daily Doses of Vitamin D – Slovenian Guidelines
Because Vitamin D deficiency is extremely common in Slovenia and routine testing is significantly more expensive than general supplementation, guidelines do not recommend screening, except for women with multiple risk factors who do not take Vitamin D. Research shows that many pregnant women in Slovenia have insufficient Vitamin D levels, so typical doses in multivitamins are often too low.
✅ For all women:
→ 800–1000 IU of Vitamin D per day
(starting when planning pregnancy, throughout pregnancy, and during breastfeeding)
✅ Women with overweight or obesity:
→ 1600–2000 IU/day (double dose)
✅ Women with risk factors for deficiency:
→ normal body mass: 1000–2000 IU/day
→ obesity: 2000–4000 IU/day
🚫 Not recommended:
bolus doses once a week or once a month
intermittent intake at longer intervals
Toxicity and Teratogenicity
Excessive amounts of Vitamin D, with long-term very high intake, can cause elevated blood calcium, which can strain the kidneys.
However, this only occurs with very high doses, which are rare in practice.
Hypercalcemia is possible in adults with:
intake of >10,000 IU/day for several months
or a single dose of >300,000 IU
Potentially toxic serum levels >375 nmol/L – which is practically very difficult to achieve with ordinary supplements.
In studies on pregnant women, up to 4000 IU/day, no side effects were reported, and teratogenic effects in humans have not been detected to date, even in exceptional cases such as 200,000 IU/day.
Upper safe limit: 4000 IU/day (also for pregnant women)
Conclusion
Vitamin D is an important part of prenatal care. Although not all clinical effects have been fully confirmed in RCTs, supplementation during pregnancy is safe, sensible, and recommended due to the high prevalence of deficiency in Slovenia, positive effects on calcium homeostasis, immune tolerance, and possible beneficial effects on SGA and some pregnancy complications.
We also encourage women to:
a balanced diet with sufficient calcium content,
sun exposure,
and regular outdoor physical activity.
