Vitamin D is naturally occurring nutrient manufactured by skin when exposed to sunlight (UVB). It is safe to continue taking vitamin D during pregnancy (and when breast feeding), in fact it is strongly recommended for all pregnant women to take vitamin D supplements as there are many benefits for the mother and baby’s well being. It is also vital in reducing the MS risk for the child. Children of people with MS have around 30x to 40x the risk of getting MS so it is important to maintain their vitamin D levels.
You may be tested by your doctor during your pregnancy if you are in an at risk group for vitamin D deficiency - this includes women with MS. Make sure that you check the amount of vitamin D in any prenatal vitamins you are taking, and factor this in with your vitamin D dosage.
Oral supplementation of cholecalciferol (maximum dose of 4,000 units daily) started less than 16 weeks gestation has been shown to be safe in pregnancy - Hollis BW, Johnson D, Hulsey TC, Ebeling M, Wagner CL. Vitamin D supplementation during pregnancy: double-blind, randomized clinical trial of safety and effectiveness. J Bone Miner Res. 2011;26(10):2341-57.