Background: The updated Endocrine Society guidelines from 2024 recommend empiric vitamin D supplementation during pregnancy to potentially reduce the risk of adverse outcomes such as preeclampsia, preterm birth, small-for-gestational-age infants, and neonatal mortality. However, the recommendation is based on low certainty evidence and the optimal dosage and its direct impact on maternal and fetal health remain uncertain, underscoring the need for large randomized controlled trials to evaluate the efficacy of higher-dose vitamin D supplementation during pregnancy.

Aim: The GRAVITD trial was designed to evaluate whether high-dose vitamin D supplementation (90 µg/3,600 IU) improves pregnancy outcomes compared to the currently recommended dose (10 µg/400 IU). Key outcomes include rates of preeclampsia, fetal growth restriction, and gestational diabetes. The trial also investigates the effects of vitamin D on placental development.

Methods: In total, 2,000 pregnant women were enrolled at the end of the first trimester and randomized 1:1 to receive either 10 µg or 90 µg of vitamin D daily. Of these, 1,854 participants completed the trial. Blood samples were collected at enrollment (n=1,974) and third trimester (n=967) to measure 25-hydroxyvitamin D [25(OH)D]concentrations. At delivery, umbilical cord blood and placentas were collected. Pregnancy and delivery outcomes were extracted from medical records and independently verified by two investigators.

Results:  At the end of the first trimester, 42% of pregnant participants had 25(OH)D levels below 75 nmol/L. High-dose vitamin D supplementation significantly improved maternal vitamin D status, with 97.5% of women in the 90 µg group reaching levels ≥75 nmol/L, compared to 76.5% in the 10 µg group. Furthermore, analysis of umbilical cord blood showed that 51% of neonates in the 10 µg group were vitamin D deficient (25[OH]D <50 nmol/L), whereas this was reduced to 11% in the 90 µg group. Further findings on pregnancy outcomes will be presented at the conference.
Perspectives: A follow-up study, GRAVITD-KID, has been initiated to assess the long-term effects of prenatal vitamin D supplementation on child health.