Lubchenco and coworkers (1963) published detailed comparisons of gestational ages with birthweights to derive norms for expected fetal size at a given gestational week. Battaglia and Lubchenco (1967) then classiied smalor-gestational-age neonates as those whose weights were below the 10th percentile for their gestational age. Low-birthweight newborns who are small for gestational age are often designated as having etalgrowth restriction. Such infants were shown to be at increased risk for neonatal death. For example, the mortality rate of SGA neonates born at 38 weeks was 1 percent compared with 0.2 percent in those with appropriate birthweights.
Importantly, manyneonates with birthweights < 10th percentile are not pathologically growth restricted, but instead are small simply because of normal biological factors. s many as 70 percent ofsuch SGA infants have normal outcomes and are thought to be appropriately grown when maternal ethnic group, parity, weight, and height are considered (Unterscheider, 2015). These small but normal infants also do not show evidence ofthe postnatal metabolic derangements commonly associated with deficient fetal growth. Moreover, intrinsically SGA newborns remain significantly smaller during surveillance to 2 years compared with appropriate-for-gestational age neonates, but they do not show diferences in measures ofmetabolic risk (Milovanovic, 2012).
Because of these disparities, other classifications have been developed. Usher and McLean (1969) suggested that fetal growth standards should be based on mean weights-for-age, with normal limits deined by ±2 standard deviations. This definition would limit SGA infants to 3 percent of births instead of 10 percent. In a population-based analysis of 122,754 births at Parkland Hospital, McIntire and colleagues (1999) showed this definition to be clinically meaningful. Also, as shown in
Figure 44-3, most adverse outcomes are in newborns smaller than the 3rd percentile. The importance ofthis cut-ofhas been independently confirmed in a prospective study by Unterscheider and colleagues (2013a).
More recently, individual or customized fetal-growth potential is proposed to replace a population-based threshold. In this model, a fetus that deviates from its individual optimal size at a given gestational age is considered either overgrown or growth restricted (Chiossi, 2017). Such optimal projections are based on maternal race or ethnicity. But, the superiority of customized growth curves has not been established (Chiossi, 2017; Costantine, 2013; Grobman, 2013; Zhang, 2011).
aneuploidy typically have disproportionately large head sizes and thus are asymmetricaLy growth restricted, which is contrary to contemporaneous thinking (Nicolaides, 1991). Moreover, most preterm neonates with growth restriction due to preeclampsia and associated utero placental insuiciency are found to have more symmetrical growth impairment-again, a departure from accepted principles (Salaia, 1995).
More evidence of the complexity of growth patterns was presented by Dashe and associates (2000). These investigators analyzed 8722 consecutive liveborn singletons who had undergone sonographic examination within 4 weeks of delivery. Although only 20 percent of growth-restricted fetuses demonstrated sonographic head-to-abdomen asymmetry, these fetuses were at greater risk for intrapartum and neonatal complications. Symmetrically growth-restricted fetuses were not at increased risk for adverse outcomes compared with those appropriately grown. These investigators concluded that asymmetrical fetalgrowth restriction represented signiicantly disordered growth, whereas symmetrical growth restriction more likely represented normal, genetically determined small stature.
Other data further challenge the concept of brain sparing.