Importance Observational studies have reported associations between pregnancy vitamin D levels and offspring cognition, but no randomized clinical trial (RCT) has investigated the effect of high-dose vitamin D 3 supplementation in pregnancy on offspring cognition into middle childhood. Objective To assess whether high-dose vitamin D 3 supplementation during pregnancy is associated with improved offspring cognition at age 10 years. Design, Setting, and Participants This is a post hoc secondary analysis of the blinded, placebo-controlled Copenhagen Prospective Studies on Asthma in Childhood 2010 RCT conducted from March 4, 2009, to November 17, 2010. The cohort included 700 mother-child pairs from Denmark, of whom 623 were randomized. Exclusion criteria were vitamin D intake above 600 IU/d; endocrine, kidney, or heart disease; and insufficient Danish language proficiency. Cognitive assessments were conducted for the offspring at age 10 years, excluding those born at less than 28 weeks’ gestation and with birth weight less than 1500 g. Participants were unblinded at 3 years of age, while investigators remained unaware of group assignments. The 10-year visits were conducted from February 11, 2019, to December 13, 2021. Statistical analyses were conducted from February to June 2025. Intervention High-dose (2800 IU/d) or standard-dose (400 IU/d) vitamin D 3 from pregnancy week 24 to 1 week post partum. Main Outcome and Measures The primary outcome was cognitive functioning across 11 functions assessed at age 10 years using a comprehensive neuropsychological test battery as part of the Copenhagen Prospective Study on Neuro-Psychiatric Development (COPSYCH). Results The vitamin D 3 RCT and the COPSYCH visit were completed by 498 children (mean SD age, 10.3 0.4 years; 258 boys 51.8%): 247 prenatally exposed to high-dose vitamin D 3 and 251 prenatally exposed to standard-dose vitamin D 3 . Covariate-adjusted analyses of standardized scores (mean SD, 0 1; higher values indicating better performance) showed positive associations of high-dose vitamin D 3 with verbal memory (β = 0.17 SD; 95% CI, 0.03-0.32 SD; P = .02), visual memory (β = 0.24 SD; 95% CI, 0.06-0.42 SD; P = .01), and flexibility or set shift (β = 0.19 SD; 95% CI, 0.01-0.37 SD; P = .04); however, high-dose vitamin D 3 was no longer associated with flexibility or set shift after multiple test correction. Conclusion and Relevance This post hoc analysis of an RCT suggests that high-dose vitamin D 3 supplementation in pregnancy is positively associated with visual memory, verbal memory, and flexibility or set shift among offspring measured at age 10 years. These findings strengthen evidence on the association of prenatal vitamin D exposure with childhood cognition. Trial Registration ClinicalTrials.gov Identifier: NCT00856947
Frederiksen et al. (Mon,) studied this question.