From: Chapter 17: Pre-Foaling Management

A mare should be brought inside at night beginning 30 to 45 days before her due date. This is done for two reasons. The first is so she can become comfortable with the surroundings and feel that the foaling stall is a safe, private place. Mares that are not at ease might delay foaling and prolong their labor until they feel more secure. Such a delay can lead to complications.

The second reason is to introduce the mare to all the local pathogens, giving her time to build immunity and concentrate this immunity in her first milk, or colostrum. The foal receives all its immunity for the first three or so months of its life via the antibodies absorbed from the mare’s colostrum during the first 24 hours of life. By introducing the mare to the local organisms ahead of time, you are, in effect, protecting the foal.

The foaling stall should be a minimum of 14 x 14 feet, have solid walls that rise at least three feet from the floor, be free of any sharp edges, and be well ventilated but draft-free. The stall should be kept clean, dry, and well-bedded at all times. The mare should be isolated from transient horses (show and sale horses) and young horses (weanlings and yearlings) to avoid exposing her or her unborn foal to any new and/or particularly virulent pathogen (rhinopneumonitis in particular). Ideally, foaling mares live in a separate barn and get turned out during the day in small compatible groups of five or six