According to the American Pregnancy Association - it is important that pregnant woman get their fair share of Vitamin D. Here is what they say:

"You can begin by making an effort to eat more vitamin D containing foods. Also sensible sun exposure (usually 5-10 min of exposure of the arms and legs or the hands, arms, and face, 2 or 3 times per week) can help as well.

However, the best way to really ensure adequate vitamin D is through simple supplementation. When supplementing, your choices will be between two forms of vitamin D. Ergocalciferol is the vegetarian form of vitamin D and cholecalciferol is the animal-sourced form, usually derived from fish liver oil or lanolin from sheep."