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Newborn infants are classified according to birth weight as small, average, or large for gestational age (Table 49-1).
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Small-for-Gestational-Age
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Small-for-gestational-age (SGA) infants are a heterogeneous group of infants who are smaller than normal at birth due to genetic or constitutional conditions, diseases such as congenital infections, or growth restriction from a smaller or poorly functioning placenta that reduces oxygen and nutrient supplies to the fetus. SGA infants are commonly defined as having a birth weight less than the 10th percentile of a population-specific birth weight versus gestational age relationship. They also can simply be thinner than normal, in which case their weight-to-length ratio (or the ponderal index = [weight, g]/[length, cm]3) is less than normal. Being thinner as an SGA infant most commonly is the result of late gestation nutritional deficiency due to placental insufficiency. It is important that all anthropometric measurements (weight, length, head circumference, ponderal index), not just weight, are used to define fetal growth as deviations in each of these can represent unique causes and specific adverse outcomes.
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Intrauterine Growth Restriction
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Intrauterine growth restriction (IUGR) is defined as a rate of fetal growth that is less than normal for the population and for the growth potential of a specific infant. Infants with IUGR, therefore, can be SGA or simply smaller than they could have been. The latter point is important because adverse outcomes of growth restriction are due to the processes that produce slower growth and the fetal adaptations to them, not just whether the infant is less than the 10th percentile in anthropometric measurements. Infants with moderate or severe IUGR tend to have asymmetric growth restriction (ie, growth restriction of muscle, fat, and organs is greater than that of the brain and the long bones). Importantly, however, such infants do have growth restriction of the brain, which can include fewer neurons, shorter axons, fewer dendrites, and less dendritic arborization and synapse formation. These adverse conditions can lead to limited cognitive capacity and behavioral problems later in life. Particularly at risk for developmental complications are very preterm infants who already are growth-restricted at birth. Constitutionally small infants (from normal but small mothers who have small ...