Productivity of hill ewe flocks in Britain is dependent on both the genotype and viability of lambs born, with birth weight being a significant determinant of their neonatal viability and subsequent growth. Hill ewes lambing in late Spring frequently endure conditions of moderate to severe feed restriction during early pregnancy in their coincident winter environment. The extent to which such restriction has a negative impact on lamb birthweight, especially where the sire genotype has been selected for improved lean tissue growth, or is offset by safe mobilisation of maternal body reserves will be crucial to subsequent survival of the offspring. Nutrient demands imposed by the foeto-placental unit(s) depend not only on stage of gestation but also on litter size. They may also be influenced by the genetic potential for muscle growth of the foetal genotype, the expression of which is influenced by early pregnancy maternal nutrition (Fahey et al., 2005). In the present study we examined whether provision of a diet meeting 0.5 versus 1.0 times ME requirements for maintenance of pregnant ewes up to Day 95 of gestation influenced the birth weights of lambs. A total of 134 ewes lambed, 30 of these with twins. Sires used were Texel rams that were either carriers or non-carriers of the Inverdale fecundity gene (FecXI) gene.