Gestation in sheep varies from 142 to 152 days with the average being 147 days. Just like people, individual pregnancies can vary, gestation periods of 138 to 159 days are not unheard of. Ewes carrying multiple lambs tend to have shorter gestations while male lambs (and heavy birth weight lambs) are usually carried for longer than female lambs. The form below calculates the lambing date based on a 147 day gestation period.
For more information on Sheep gestation, please check out the Sheep 201 site.