This review will focus on the evidence for in utero effects on development of skeletal muscle in farm and laboratory animals, particularly sheep and pigs. We will describe genetic and environmental factors that have been shown to alter the numbers of muscle fibres formed and outline our working hypothesis for the mechanism involved and the critical window during pregnancy when these effects are seen. We will then discuss the long-term consequences in terms of body composition. Although this review concentrates on skeletal muscle development, the mechanism we suggest might be equally applicable to other tissues in the body (e.g. the brain, kidneys or sex organs) and, therefore, impact on their physiological functions.