University of wisconsin-madison

Length of Gestation

The gestation period of the mare should be documented because she will tend to repeat that gestation length. Thus in future years this is valuable in predicting the day of foaling. Normal gestation in the mare is 335-342 days although gestion lengths from 320 to 367 occur. Foals born before 320 days are considered premature and require special veterinary care if they are to survive. Foals born before 305 days do not survive. Long gestations of up to a year appear to pose no problem, the foal will be born when it is time.

Gestation length is effected by many factors including day length and genetics. Mares due to foal in late winter to early spring may have gestations 5 - 10 days longer than mares foaling later in the breeding season. This can cause a problem when trying to breed mares from mid-Feburary to early March. This problem can be overcome by placing these pregnant mares on 18 hours light, December 1. This treatment which is the same as used to make mares cycle early will shorten gestation by 10 days. Remember this if you place all your mares on artifical light to induce early ovulation in non-pregnant mares. Even mares whose parturition is due later in the breeding season will have gestations advanced by these conditions.