Supplementing vitamin D is recommended by a few calm agencies and is reasonably sensible. Especially if you don't get much sun. UK children under 5 are recommended to take a multivitamin of A, C and D.
We're at the frustrating stage of having credible methods of action, but weak evidence for efficacy, for some vitamin interventions.
> The number of trials and outcomes reported are too limited, and in general are of low quality, to draw conclusions on the usefulness and safety of this intervention as a part of routine antenatal care. Further rigorous randomised trials are required to evaluate the role of vitamin D supplementation in pregnancy.
We're at the frustrating stage of having credible methods of action, but weak evidence for efficacy, for some vitamin interventions.
(http://summaries.cochrane.org/CD008873/vitamin-d-supplementa...)
> The number of trials and outcomes reported are too limited, and in general are of low quality, to draw conclusions on the usefulness and safety of this intervention as a part of routine antenatal care. Further rigorous randomised trials are required to evaluate the role of vitamin D supplementation in pregnancy.