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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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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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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; ...