As winter ends and spring begins, metabolism increases. It can be taxing for deer, especially does carrying young, because their bodies are working to fuel the development of a fetus, as well as maintain their own health. Nutritional needs increase as their fetuses grow, and in some habitats, that can lead to problems. If deer must use as much energy to forage for food as they acquire through consuming the food they find, there's no benefit. Health will decline.
Deer living in good habitat at worst tend to be only mildly affected by winter.
Generally, though, a well-nourished doe will come through winter in healthy condition. Her strong body will provide proper nourishment for her fawns, which will, in turn, also be healthy. After birth, the fawns will be heavier, larger, and stronger than those borne by malnourished females. A properly nourished doe also will provide enough milk to nourish and satisfy the demands of the newborn fawns, whereas malnourished does will have trouble producing the quantity of milk her fawns need. One of the most critical times for unborn fawns is yet to come.