Department of Social Medicine, University of Birmingham, U.K. Introduction Following Ylppö's suggestion that infants weighing less than 2500 g should be considered not mature (1920), birth weight was used as an index of maturity. This practice was open to criticism on two grounds : as an administrative device for identifying vulnerable infants, the limit of 2500 g was too high; and since low birth weight may result from either or both retarded fetal growth and early onset of labour, identification of maturity with weight confused two different classes of problems (McKeown & Gibson, 1951). Subsequently, by international agreement, the definition of prematurity was changed. But in the meantime another term had appeared in the literature; intrauterine growth retardation began to be referred to as an entity, in much the same way as was premature birth in the earlier period. The new term certainly distinguished low weight due to retarded fetal growth