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Small for Gestational Age

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Newborn infants are classified according to birth weight as small, average, or large for gestational age (Table 47-1).1,2 Small-for-gestational-age (SGA) infants are a heterogeneous group of infants who are smaller than normal at birth because of genetic or constitutional conditions, diseases, or nutrient insufficiency. SGA infants have a birth weight less than the 10th percentile of a population-specific birth weight versus gestational age. Broader definitions include infants whose weight percentile is less than that for length and head circumference (eg, weight at 25th percentile but length and head circumference at 75th percentile). In this case, the weight-to-length ratio—or the ponderal index = (weight, g)/(length, cm)3—is less than normal, indicating that growth rates of visceral organs, adipose tissue, and skeletal muscle, the principal determinants of weight, were less than that of length. A low weight-to-length ratio occurs most commonly in fetuses with late gestation nutritional deficiency, usually a result of placental insufficiency.

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Table 47-1. Classification of Fetal Growth
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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.2 IUGR infants therefore can be small for gestational age or simply smaller than they could have been but still with weights and other anthropometric indices within the normal range.4 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 these, not just whether the infant is less than the 10th percentile. Nearly any aberration of biological activity in the placenta and/or fetus can lead to fetal growth failure.5 Moderately and severely IUGR infants tend to have asymmetric growth restriction; that is, body growth restriction is greater than brain growth restriction. The degree of asymmetry varies considerably depending on the duration and severity of the growth inhibition. Constitutionally small infants (from normal but small mothers who have small uteruses) tend to have more symmetrically restricted brain and body growth. Asymmetric and symmetric growth restriction therefore represent the 2 extreme patterns of abnormally slow fetal growth rates.

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Low Birth Weight

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Most infants with low birth weights are the result of a shorter than normal gestation (ie, they are preterm). Also, a large fraction of preterm infants are growth restricted; ...

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