The Book of Household Management by Mrs. Beeton

March, under the artificial system, so much pursued now to please the appetite of luxury, lambs can be procured at all seasons. When, however, the sheep lambs in mid-winter, or the inclemency of the weather would endanger the lives of mother and young, if exposed to its influence, it is customary to rear the lambs within-doors, and under the shelter of stables or barns, where, foddered on soft hay, and part fed on cow's milk, the little creatures thrive rapidly: to such it is customary to give the name of House Lamb, to distinguish it from that reared in the open air, or grass-fed. The ewe goes five months with her young, about 152 days, or close on 22 weeks. The weaning season commences on poor lands, about the end of the third month, but on rich pasture not till the close of the fourth--sometimes longer.