So how do deer know when to have their fawns? Well since gestation (pregnancy) length is about 200 days in whitetails, it means that the timing of breeding actually controls when fawns are born. Breeding is mainly controlled by photoperiod (day length) and genetics, but adult buck to doe ratio can also greatly affect when fawns are born. In many areas of the whitetail's range, there are too many does than bucks and this can have profound effects on the herd.
If the ratio is too skewed towards does, there likely won't be enough bucks to breed all the does coming in to estrus during the rut, causing the does to come back into estrus again 28 days later, and the cycle repeats until she is bred. This causes fawning to occur over a longer period of time, which can cause issues.