Drs Zhang, Merialdi, Platt, and Kramer have provided readers of the American Journal of Obstetrics and Gynecology with a benchmark article, one that should be read and taken to heart by all who care for pregnant women and newborn infants. In this review, they shine a clarifying light on 3 decades of increasing confusion in the language and concepts that are used to describe fetuses and infants who are, or are suspected of being, small. Once upon a time, there were 3 categories that were defined by percentiles of growth that encompassed the entire population of newborn infants: (1) infants with birthweights <10th percentile, called small for gestational age (SGA); (2) infants with birthweights from 10th to 90th percentile, called appropriate for gestational age, and (3) infants with birthweights of >90th percentile, called large for gestational age (LGA).

These categories were not thought of as diagnoses, just labels that facilitated further evaluation at a time when consideration of birthweight relative to gestational age was a new concept. Reasons for being small or large were considered separately. Physicians who cared for these infants memorized lists of reasons that a newborn infant might be SGA or LGA, with the understanding that some were normally small or large and others were smaller or larger than they should have been. The newborn concept of SGA was later applied to the fetus as intrauterine growth retardation, now called intrauterine or fetal growth restriction (FGR). As this concept entered fetal care, the definition of SGA that was population-based (<10th percentile of all fetuses of the same gestational age) was conflated with a standard-based (<10th percentile of healthy fetuses of the same gestational age) definition of FGR that included a fetus of any size who has not achieved its optimal growth potential, regardless of growth percentile. The importance of this difference is the subject of the review by Dr Zhang and his coauthors. To quote,

“A population reference is often established on the basis of a large sample size (ideally representing the underlying population) with a study population that includes both low-risk and high-risk pregnancies and both normal and abnormal perinatal outcomes … a standard usually is based on low-risk pregnancies with a normal outcome. When the “population reference” and the “standard” are applied to an individual fetus or infant, interpretation of the findings differs. Use of a population reference will yield a relative fetal size in relation to the total population; a standard will assess a fetal size in comparison with normally grown fetuses. Thus, a standard may have more clinical utility than a population reference.”

Zhang et al point out that the consequences of confusing the 2 concepts are over-diagnosis of pathologic condition and consequent unnecessary interventions among small normally grown fetuses, and failure to identify and intervene on behalf of poorly grown fetuses whose weight is estimated to be >10th percentile for age. Both are common but unacceptable and could be reduced by clarification of the distinction. The authors believe that current attempts to improve the detection and care of these fetuses through customized fetal growth profiles and integration of additional markers represent improvements but fall short in the United States because of the diversity of the population. Until the improvements that they describe become reality, I would like to propose that the conflation of definitions be resolved by applying the term SGA to all infants and fetuses whose weight falls at <10th percentile for gestational age (population-based) and by limiting the FGR designation to infants and fetuses whose growth is suspected to be less than optimal, recognizing that SGA infants are not all FGR and that FGR infants are not all SGA. SGA would be based on growth percentiles, and FGR would be based on evidence of pathologic growth. I believe this integration of obstetric and pediatric terminology could improve the antenatal, intrapartum, and neonatal care of small babies.