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Weight at delivery once was considered evidence of prematurity (birthweight <2500 g) or postmaturity (macrosomia; birthweight >4500 g). These criteria later were revised upon the realization that birthweight can be reflective of other pathologic processes aside from prematurity. Abnormalities in fetal growth at each end of the spectrum—both large for gestational age fetuses and fetuses with suspected intrauterine growth restriction—are associated with an increased risk of adverse perinatal outcome. Normative standards applying to such ultrasound parameters as estimated fetal weight, abdominal circumference, and head circumference (HC) were developed.

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Abnormal fetal growth or fetal size is most commonly defined on the basis of estimated fetal weight. Fetuses with an estimated fetal weight ≤ the 10th percentile are classified as having intrauterine growth restriction (IUGR), and those with a weight ≥ the 90th percentile are classified as large for gestational age (LGA). Both IUGR and LGA fetuses have increased risk for perinatal morbidity and mortality (Tables 16–1 and 16–2). The pathogenesis, differential diagnosis, and treatment are different for the 2 extremes of growth.

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Table 16–1. Some Complications Associated with Intrauterine Growth Restriction Pregnancy.
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Table 16–2. Some Complications of Large for Gestational Age Pregnancy.
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Essentials of Diagnosis

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  • Estimated fetal weight less than the 10th percentile for gestational age on ultrasound

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Many terms have been used to describe fetuses with disproportionately small growth. Intrauterine growth restriction (IUGR) is used to designate a fetus that has not met its growth potential and is defined as estimated fetal weight (EFW) at or below the 10th percentile for gestational age. Small for gestational age (SGA) is a term that applies to the infant that is less than the 10th percentile at birth.

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Pathogenesis

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Approximately 70% of fetuses with EFW below the 10th percentile are simply constitutionally small; thus, the term IUGR is inaccurate for many fetuses. Distinguishing between normal and pathologic growth can be difficult, but a fetus with normal anatomy, normal amniotic fluid volume, and normal growth pattern over time will generally be constitutionally small rather than IUGR. Some nonpathologic factors that ...

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