A lot of people do breed so the foal is born in Jan. So the foal is bigger for the weanling classes. You hould be fine as long as you keep the mare in a fully inclosed stall with plenty of straw bedding. A foal blanket will really help keep them warm but just besure to have a waterproof one. What we do for our winter babies is we have this thick polar fleece under blankets and then get a thick waterproof winter blanket to put on top. A lot of times people think they need to put a heat lamp in the stall...but don't it's really dangerous. Also, if you are present for the birth, heat some towels up in the dyer and towel dry the foal if the mare allowes.