Note:  There can be a seasonal effect with mares due to foal in late winter and early spring which may result in carrying their foals approximately 5 to 10 days longer than mares foaling late spring or summer.  Exposing pregnant mares to artificial lighting regimens can reduce gestation by an average of ten days.  There are many other factors that may influence gestational length include sex of the foal, maternal nutrition, ingested toxins and environmental stresses.