The hypothesis tested in this experiment was that Merino lambs with lower birthweights, due to poor ewe nutrition during pregnancy, would have more fat and less muscle than Merino lambs with higher birthweights. At two sites (Victoria and Western Australia) in each of 2 years, a wide range in the liveweight profiles of ewes was generated during pregnancy and lactation by varying the amount of supplements fed and feed on offer grazed. Progeny had fat and muscle depth at the C-site measured at various ages from 8 to 28 months of age. Across the four experiments, there were differences of 0.5 kg in birthweights and 8 kg in weaning weights between extreme treatments. The effects on the depth of fat and muscle were very small with fat depth increasing by 0.1-0.2 mm (5-7%) and muscle depth increasing by 0.2-0.7 mm (1-3%) when birthweights decreased by 1 kg. The effects of birthweight on fat depth are consistent with our hypothesis whereas the effects of birthweight on muscle depth are in contrast to our hypothesis. Nevertheless, the impacts of birthweight on the depth of fat and muscle measured at the C-site of progeny from Merino ewes, is unlikely to be of any commercial significance within the range of nutritional scenarios during pregnancy and lactation that are likely to be experienced within the Australian sheep industry. © CSIRO 2013.