Even marginal AFI values (5–10 cm) have been associated with doubling of the risk of adverse perinatal outcome.23
- When to test. We lack randomized trials of amniotic fluid volume assessment in high-risk pregnancies, but observational studies suggest that weekly determination of AFI is helpful in sonographically identified growth-restricted fetuses.
A low or marginal AFI should be followed by more frequent examinations. Even a marginal AFI may be followed by twice-weekly surveillance.
Biophysical profile effectively predicts adverse perinatal outcome, with a false-negative rate of 0.8 per 1,000, negative predictive value of 99.9%, and a false-positive rate of 40% to 50%.24 A low BPP is associated with fetal hypoxia and acidosis, and a declining BPP reflects progressive worsening of the fetal condition.
In FGR, a significant association exists between abnormal Doppler indices and fetal acid-base compromise.
Although equal weight was originally accorded all the parameters of the profile, experience suggests that the independent risk of oligohydramnios warrants immediate reassessment of the management plan.
Like the nonstress test, the BPP has a low false-negative rate and a high false-positive rate. Four randomized trials of the test have been conducted, involving about 2,800 patients, with no confirmation of its effectiveness. A Cochrane systematic review25 concluded that this cumulative sample size is insufficient to draw definitive conclusions.
The test remains a standard, however, and is recommended to confirm fetal wellbeing or as part of a comprehensive test panel in significantly high-risk pregnancies.
- When to test. Initially perform the test once a week, increasing to twice weekly when the severity of growth restriction warrants.
Daily testing may be indicated in complications such as severe growth restriction and pregestational diabetes. In early preterm pregnancies (less than 32 weeks) with nonreassuring NST or umbilical arterial Doppler, BPP can help guide optimal management.
Umbilical arterial Doppler ultrasound is a powerful predictor of adverse perinatal outcomes in high-risk pregnancies.
Doppler indices include the pulsatility index, systolic/diastolic ratio, diastolic average ratio, and the resistance index. Of these, the last demonstrated the greatest ability to predict abnormal perinatal outcomes.
A prospective, blinded study26 in highrisk pregnancies demonstrated significant diagnostic efficacy with a sensitivity of 79%, specificity of 93%, positive predictive value of 83%, negative predictive value of 91%, and kappa index of 73%. The last value is consistent with a good to excellent diagnostic test.
A significant association also exists between abnormal Doppler indices and fetal acid-base compromise in FGR. The absence of enddiastolic flow (FIGURE) is associated with markedly adverse perinatal outcome, particularly a high perinatal mortality rate, malformations, and aneuploidy. Nicolaides and coworkers27 found that, when the end-diastolic velocity was absent, 67% to 80% of the fetuses were hypoxic and 45% were acidotic. Reversal of end-diastolic flow is associated with an even worse prognosis. The adverse consequences of absent or reverse end-diastolic flow are listed in TABLE 2.
In contrast to the NST and BPP, the effectiveness of fetal surveillance with umbilical arterial Doppler ultrasound in improving perinatal outcome in high-risk pregnancies has been confirmed by extensive randomized trials and their meta-analyses.28,29 These studies provide compelling evidence that, when used in well-defined high-risk pregnancies, especially those involving FGR or preeclampsia, umbilical arterial Doppler sonography reduces perinatal death, cesarean delivery for fetal distress, elective deliveries, and antenatal admissions.
Doppler sonography of the fetal cerebral and venous circulation also helps identify progressive fetal compromise in high-risk pregnancies, including those with FGR. When the fetal condition is deteriorating, cerebral arteries dilate to compensate, and pulsations appear in the umbilical vein with reversal of flow in the ductus venous.
These signs of grave fetal status indicate immediate delivery.
FIGURE 1 Duplex pulsed Doppler
Sonogram showing absent end-diastolic flow in the umbilical artery of a growth-restricted fetus.TABLE 2
Perinatal outcomes for absent and reversed end-diastolic velocity in the umbilical artery
| PERINATAL OUTCOME | MEAN | RANGE |
|---|---|---|
| Mortality | 45% | 17–100% |
| Gestational age | 31.6 weeks | 29–33 weeks |
| Birth weight | 1,056 g | 910–1,481 g |
| SGA | 68% | 53–100% |
| Cesarean section for fetal distress | 73% | 24–100% |
| Apgar score <7 at 5 minutes | 26% | 7–69% |
| Admission to NICU | 84% | 77–97% |
| Congenital anomalies | 10% | 0–24% |
| Aneuploidy | 6.4% | 0–18% |
| SGA = small for gestational age; NICU = neonatal intensive care unit | ||
| Reprinted with permission from: Maulik D, ed. Doppler Ultrasound in Obstetrics and Gynecology. | ||
| New York: Springer-Verlag; 1997:364, Table 21.2. | ||
Assessing the fetus for malformation and aneuploidy
Because of the association between FGR and fetal malformations and aneuploidy, the fetus should be assessed for these complications, especially when growth restriction is severe, develops early, and is not associated with hypertensive disorders, oligohydramnios, or abnormal umbilical arterial Doppler indices.
Most etiologies are either not amenable to therapy or fetal growth is not improved by treatments that benefit the mother.
The current standard practice is aneuploidy screening in early pregnancy and fetal anatomical scanning at midgestation. If significant risks of aneuploidy are present, a fetal karyotype is recommended, along with appropriate counseling.