GESTATION CALCULATOR The average length of gestation for horses is 320 – 360 days. Use the calculator below to get the date 340 days from her last service/or the date she ovulated. Date Mare Was Bred Estimated Gestation Period Calculate Due Date Tetanus Vaccination Due Your pregnant mare should be vaccinated for tetanus (Equivac T) approximately 30 days prior to her due date. Deworming Due Your pregnant mare should be dewormed with a macrocyclic lactone dewormer which is safe for use in pregnant mares (eg.Equimax).
