Assessment of gestational age is important in early preg-nancy, because dating becomes increasingly imprecise at later gestational ages.Antenatal recognition of IUGR depends upon the recognition of risk factors and the clinical assessment of uterine size, fol-lowed by biometric measurements.
Physical examination is limited in usefulness in recognizing IUGR or in making a specific diagnosis, but it is an impor-tant screening test for abnormal fetal growth. Maternal size and weight gain throughout pregnancy also have limited value, but access to such information is readily available; a low maternal weight or little or no weight gain during pregnancy may suggest IUGR. Serial measurements of fundal height are commonly used as a screening test forIUGR, but have high rates of false-negative and false-positive predictive values. Between 20 and 36 weeks of ges-tation, fundal height should increase approximately 1 cm per week, consistent with gestational age in weeks (Fig. 18.1). A discrepancy may be related to constitutional factors, but a significant discrepancy of more than 2 cm may indicate IUGR and the need for an ultrasound examination. Clinical estimations of fetal weight alone are not helpful in diagnos-ing IUGR, except when fetal size is grossly diminished.
If IUGR is suspected based on risk factors and clinical assessment, ultrasonography should be performed to assess fetal size and growth. Specificfetal biometry measurementsare compared with standardized tables that reflect normal growth at a certain gestational age. The four standard fetal measurements include the (1) biparietal diameter, (2) head circumference (HC), (3) abdominal circumference (AC), and (4) femur length. Conversion of individual morphologic measurements to fetal weight using published equations or ratios of measurements can provide useful estimations of fetal size. An abdominal circumference within the normal range reliably excludes growth restriction, with a false-negative rate of less than 10%. A small abdominal circum-ference or fetal weight estimate below the 10th percentile suggests the possibility of growth restriction, with the likelihood increasing as the percentile rank decreases.
When IUGR is suspected, serial measurements of fetal biometric parameters provide an estimated growth rate. Suchserial measurements are of considerable clinical value in confirming or excluding the diagnosis and assessing the progression and severity of growth restriction. Given the high incidence of genetic and structural defects associated with IUGR, a detailed ultrasound survey for the presence of fetal structural and functional defects may be indicated.
Following recognition of altered fetal growth, a search for potential etiology should ensue. Ultrasonography should in-clude a detailed anatomic survey to evaluate for the pres-ence of structural anomalies, given the high incidence of genetic and structural defects with IUGR. Ultrasound eval-uation should also include an assessment of amniotic fluidvolume. The combination of oligohydramnios (dimin-ished amniotic fluid volume) and IUGR is associated with severe disease and increased morbidity. The mechanism of decreased amniotic fluid is thought to be decreased placen-tal perfusion of oxygen and nutrients with a compensatory redistribution of fetal blood favoring the brain, adrenal gland, and heart. The consequent decrease in fetal blood to the kidneys leads to a reduction of urine output, which is the primary source of amniotic fluid in the second half of pregnancy.
Direct invasive studies of the fetus are useful in selected patients with IUGR. Amniocentesis for fetal lung matu-rity may assist delivery planning near term or when there is uncertainty regarding gestational age and concern for growth restriction. Fetal karyotyping and viral cultures and polymerase chain reactions can be performed on fluid obtained by amniocentesis. Rarely, chorionic villus sam-pling (biopsy of placenta) or direct blood sampling (per-cutaneous umbilical blood sampling) may be necessaryfor specific studies.
Doppler velocimetry of fetal vessels provides furtherinsight into the fetal response to altered growth, and has become part of the standard assessment of the fetus once IUGR is diagnosed. Doppler velocimetry has been shown to both reduce interventions and improve fetal outcome in pregnancies at risk for IUGR. Fetal-placental circula-tion is evaluated in the umbilical artery and is measured by a systolic/diastolic (S/D) ratio. The S/D indirectly mea-sures impedance or resistance downstream within the pla-cental vessels. As placental resistance increases, diastolic flow decreases and the S/D ratio rises. A normal S/D ratioat term is 1.8 to 2.0.
Fetuses with IUGR with absent or reversed diastolic flow have progressively worse perinatal outcomes (Figure 18.2). The fetal middle cerebral artery is also evaluated and reflects fetal adaptation. The patho-physiologic response to reduced placental perfusion gen-erally spares the fetal brain, resulting in an increase of diastolic and mean blood flow velocity in the middle cere-bral artery. Ductus venosus may also be evaluated by Doppler ultrasound, and the fetus with abnormal ductus flow is at very high risk of adverse outcome.