This information comes from Training your foal: Raising a foal from birth to backing by Renate Etl (2003). The pregnancy lasts 330 to 350 days, and can sometimes trail an extra 10 days beyond that! But on the big day, you will see that the mare becomes far away and withdrawn from you, turning inward to prepare for the big task.She distances herself from the routines around her, and resin drops appear on her udders. Her tummy drops and her flanks cave. She is likely to birth in the middle of the night, in the dark hours where things are safer and she is rested and all is quiet. The whole process takes about 30 minutes and then it takes the foal about a total hour to "figure it out," get on his little legs, stand, and find his mother's milk.
I know I'll be lucky if it happens, but I hope I am at the birth. I hope that I will be able to see Hailee withdraw and prepare for her miracle. But mostly, I hope that Hailee and Hailee, Jr. have an uneventful delivery.