Individual piglet birthweight is a major determinant of subsequent survival and weaning weight of the litter. Low birthweight piglets are more likely to die from starvation and thermoregulatory stress than their heavier littermates, while those that do survive grow more slowly and reach a lower mature body weight with a higher body fat to muscle ratio. Inadequately grown porcine fetuses are often characterised by assymetrical organ development although it is not clear when this difference in relative organ size arises and whether this is a consequence of changes in cell size or cell number. The objectives of this study were to assess a range of determinants of fetal growth at stages of gestation and to determine when changes in the relationship between fetal size and organ development occur.