Small for Gestational Age and Intrauterine Growth Restriction: Definitions and Standards

There is very extensive literature dealing with the definitions of the related terms intrauterine growth restriction (IUGR), fetal growth restriction (FGR), and small for gestational age (SGA). Numerous authors in many countries have published research findings attempting to define appropriate boundaries to classify growth in fetuses and infants. First, it should be emphasized that, conceptually, FGR and IUGR are not the same entity as "small for gestational age." For most authorities, a growth-restricted newborn is now defined as an infant who has not achieved its genetic growth potential in utero. This, of course, presupposes that one can determine growth potential, which, in reality, is not now possible. In many recent studies, authors have tended to use the term "small for gestational age," or "SGA," for a fetus who has failed to achieve a specific and arbitrary anthropometric or weight threshold by a specific gestational age. We are aware, however, that some SGA infants may be constitutionally small and may represent only the tail end of a normal distribution. For this article, we will focus on the definitions of SGA rather than on those for intrauterine growth restriction or fetal growth restriction, but at times will use the terms interchangeably.