Birthweight and Fetal Growth

Abstract Birthweight is a popular research topic because data are so accessible and the associations of small birthweight with infant mortality and morbidity are so strong. It is doubtful, however, that birthweight itself is the causal factor. Some babies are constitutionally small without excess risk. The underlying pathology that produces the associations between small birthweight and adverse infant outcomes is a clinical condition known as fetal growth restriction (FGR), defined in principle as the failure of a fetus to achieve its full growth potential. As yet, there is no way to measure “potential” growth. Without a gold-standard definition, fetal growth restriction is often defined as small size at birth (small-for-gestational-age, or SGA). This use of SGA is flawed and has led to misunderstandings about the epidemiologic features of fetal growth restriction. Until there is a way to identify FGR by measures beyond birthweight, the infants affected by FGR cannot accurately be identified, diagnosed, or treated—and epidemiologic studies of its causes cannot be effectively conducted.