Bone formation and loss are related to the strain imposed on bone by muscle forces. Bone mineral content (BMC) and lean mass (LM) of fetal lambs was determined at day140 of pregnancy in 8 groups of ewes which were of either large or small body size, on either high (ad lib) or maintenance pasture intake from day21 of pregnancy, or carrying either singletons or twins. BMC and LM (utilizing DXA scanning) of fetal hindquarters/spine were corrected to leg length. BMC and LM were less in twin than singleton groups (p<0.001). Large ewes on high intake produced single fetuses with a (group mean) BMC/LM ratio that was higher (p< 0.002) than that in fetuses of large ewes with singletons on maintenance intake or twins on either high or maintenance intakes, the ratios of which were not different. In single fetuses from small ewes on high intake the BMC/LM ratio was higher than those from small ewes with singletons on maintenance intake or twins on either high or maintenance intakes, the ratios of which were not different. The ratio was not different in singleton fetuses of ewes on high intake, whether they were large or small. Different fetal environments resulted in a given amount of muscle being associated with a higher or lower bone mass. Dietary intake during pregnancy was more important than maternal size in affecting the ratio. We conclude that intra-uterine environmental factors may be important in determining bone mass postnatally, and possibly later in life.