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Prenatal care seeks to diagnose pathologically small fetuses carrying an increased risk of perinatal morbidity and mortality. Intrauterine growth restriction (IUGR) occurs when fetuses are unable to achieve their genetic potential for in utero growth. To exclude healthy but constitutionally small fetuses, many clinicians define IUGR as estimated fetal weight (EFW) less than the third percentile. Others prefer to use a broader definition, often referred to as small for gestational age (SGA), which identifies fetuses with an EFW below the 10th percentile. Ultrasound-derived EFW's should be plotted against gestational age-specific growth charts. Some institutions use customized growth charts that account for local population differences and maternal factors, including height, weight, past obstetric history, and ethnicity.

IUGR can be described as symmetrical or asymmetrical. Symmetrical IUGR has traditionally been thought to reflect reduced fetal growth potential, as seen with genetic conditions, congenital anomalies, or intrauterine infection. Approximately one-third of infants with IUGR display this pattern of growth in which all biometric parameters are smaller than expected. It usually has an early onset, before 32 weeks of gestation. Asymmetrical IUGR reflects reduced fetal growth support and impaired placental transfer of nutrients. This leads to a decrease in soft tissue mass and, in particular, a decreased deposition of glycogen in the fetal liver. Because the liver is the predominant intraabdominal organ, the abdominal circumference is reduced. The fetus compensates with cerebral redistribution of blood flow to protect growth of the brain and head. This results in the typical head-sparing asymmetry in which the abdominal circumference and femur length lag behind expected. Often, asymmetric growth is not recognized until after 32 weeks' gestation. There is considerable overlap between the two forms of IUGR, and with increased duration and severity, either may attain a symmetric profile of growth restriction.

Of all fetuses with an estimated fetal weight under the 10th percentile, approximately 80 to 85 percent are constitutionally small but healthy; 10 to 15 percent have true IUGR caused by placental insufficiency; and 5 to 10 percent are affected by a pathologic condition, including aneuploidy, structural anomalies, or intrauterine infection (Fig. 33-1). Although the proportion of constitutionally small babies in a given population remains relatively stable, the prevalence of pathologic growth restriction is less predictable.

FIGURE 33-1.

Causes of fetal growth restriction.

IUGR may be detected fortuitously after an ultrasound performed for another indication or deliberately after serial assessments of a high-risk patient. Some advocate routine third-trimester growth ultrasound for all women. The benefits of this have not been substantiated and would be impossible in resource-challenged settings. Clinical palpation of the abdomen and measurement of the symphyseal-fundal height (SFH) may raise the suspicion of growth restriction but is not sufficiently sensitive or specific; both techniques identify, at best, 30 percent of fetuses with IUGR. ...

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