found this: "The breeding season for most Rocky Mountain mule deer begins in late October with fawning occurring as early as late May and peaking in June. Mule deer does have a gestation period of about 210 days. Arguably, the timing of mule deer reproduction evolved to maximize both doe and fawn survival.
Does who are bred in late October would fawn when spring weather and forage conditions were optimal. Forage plants at this time would be at their maximum protein content and digestibility. The availability of high protein forage would allow the doe to meet the energy demands of lactation. Until fawns are weaned, does will have little opportunity to build the energy reserves needed for the following rut and winter.
Fawns also require high protein diets to achieve maximize growth. Thus, the earlier a fawn is born and begins nursing, the more likely it is going to be able to take advantage of the high protein content available in spring forages. Fawns that receive adequate nutrition during juvenile growth periods are more likely to put on the body mass needed to survive winter. "