Vitamin D supplementation should start from time of conception, and continue after birth. During pregnancy, the fetus will receive adequate vitamin D from the mother, provided she is supplementing with adequate doses. To work out the appropriate vitamin D dose for your child, use the ratio of 100IU per 1kg of body weight. Provided that you follow this supplementation guideline, your child's blood levels should remain above 100nmol/L, which is felt to be the safe level to help prevent MS, and there is no need for annual blood testing.