Vitamin D intake during pregnancy is associated with attenuated Season-of-Birth effects on infants’ language.

The time of birth within the year can influence life outcomes, a phenomenon known as a "season-of-birth effect". While this effect is well documented in relation to health, its impact on early development is poorly understood. We investigated this effect in the context ofinfant language and cognitive development in Norway. In a first cross-sectional study, using parental reports of vocabulary development with 1187 infants, we found that 12-month-olds born in mid-April had lower expressive vocabulary than those born in mid-October. The second longitudinal study assessed 848 infants between 6 and 18 months of age using the Bayley Scales of Development and showed that infants born in mid-April had lower cognitive scores than those born in mid-October. Although both studies indicated lower scores for infants born in late winter, the second study also showed that regular prenatal vitamin D intake mitigated these effects, reducing season-of-birth effects for language development. These findings underscore the importance of adequate maternal vitamin D status to reduce seasonal disparities in early language development.