Tylavsky FA, Kocak M, Murphy LE, Graff JC, Palmer FB, Völgyi E, Diaz-Thomas AM, Ferry RJ. (2015) Nutrients. 7(12) 9918-30. doi: 10.3390/nu7125499.
Observational studies like these cannot provide evidence of causality. Currently, controlled trials of Vitamin D supplementation in pregnancy are very few, and have not assessed cognitive and motor development. However, these have shown improvements in maternal Vitamin D status, possible improvements in birth weight, and importantly - no adverse effects.(De Regil et al 2012)
Given the high prevalence of sub-optimal Vitamin D status during pregnancy (affecting more than 4 in every 10 mothers in the current study), more controlled trials are needed to assess the potential effects on children's mental as well as physical health and development. Meanwhile, however, existing evidence indicates that supplementation to ensure that all mothers have adequate Vitamin D status during pregnancy is safe, and may also have general health benefits for both mothers and infants.