To examine the association between cord 25-hydroxyvitamin D₂₊₃ (25(OH)D) and attention deficit hyperactivity disorder symptoms in toddlers, using Child Behaviour Checklist for ages 1.5–5.

In a population-based birth cohort, a Child Behaviour Checklist for ages 1.5–5 questionnaire was returned from parents of 1233 infants with mean age 2.7 (standard deviation 0.6) years. Adjusted associations between cord 25(OH)D and Child Behaviour Checklist–based attention deficit hyperactivity disorder problems were analysed by multiple regression.

The median cord 25(OH)D was 44.1 (range: 1.5–127.1) nmol/L. Mean attention deficit hyperactivity disorder problem score was 2.7 (standard deviation 2.1). In adjusted analyses, cord 25(OH)D levels >25 nmol/L and >30 nmol/L were associated with lower attention deficit hyperactivity disorder scores compared to levels <=25 nmol/L (p = 0.035) and <=30 nmol/L (p = 0.043), respectively. The adjusted odds of scoring above the 90th percentile on the Child Behaviour Checklist–based attention deficit hyperactivity disorder problem scale decreased by 11% per 10 nmol/L increase in cord 25(OH)D.

An inverse association between cord 25(OH)D and attention deficit hyperactivity disorder symptoms in toddlers was found, suggesting a protective effect of prenatal vitamin D.