I would definitely buy foal jackets if it happens to get bitterly cold. My filly was born at the end of April but ee still had a few days where the temperature went down into the -20s, so I did make use of her jacket a few times. Also, not sure where you are, but where I am, we have less than 7 hours of daylight by the end of December. Lots of outdoor time in the sun will be good for growth and prevent issues with vitamin D deficiency. As long as the mare's have a warm dry place to foal, they should do fine.