Fetal growth restriction

Abdominal circumference and estimated fetal weight. A review²⁰ of 60 studies found that abdominal circumference (AC) and sonographically estimated fetal weight (EFW) were the best predictors of birth weight below the 10th percentile in high-risk pregnancies. AC below the 10th percentile had sensitivities ranging from 72.9% to 94.5%, false-positive rates of 16.2% to 49.4%, and a common odds ratio of 18.4. An EFW below the 10th percentile had a common odds ratio of 39.1, which was the highest among all the biometric measurements. Its sensitivity ranged from 33.3% to 89.2%, and false-positive rates ranged from 9.1% to 46.3%.

The proportionality of fetal dimensions, such as the head/abdominal ratio, was not a good predictor, although it is routinely generated in ultrasound biometry.

Ultrasound estimation of fetal weight is based on a combination of 2 or 3 biometric measurements of the fetus, including the biparietal diameter (BPD) or the head circumference (HC), AC, and femur length (FL). Several formulae yield varying estimations of weight. Thus, a fetus identified as growth-restricted by 1 formula may not be so diagnosed by another. For this reason, it is prudent to be consistent in their use. EFW generated from measurements of the head (BPD or HC), AC, and FL is most reliable.²¹

The EFW also is expressed as the percentile for the gestational age.

Limitations of ultrasound estimation.

The optimal process of translating dimensional measurements into fetal body mass for both health and disease remains unknown. This leads to inaccurate assumptions and erroneous weight projections.

Inaccuracies also result from variations in measurement. In addition, the estimation is less accurate in the lower and upper ranges of fetal weight distribution and in the presence of oligohydramnios. As a result, estimated weight may differ from actual weight by as much as 20%, with greater margins of error at the lower and upper extremes.

Guidelines for screening and diagnosis

Assess all pregnancies for risk factors and determine the gestational age in early pregnancy, especially in women at higher risk of FGR. If the patient has substantial clinical risks or there is suspicion of growth restriction, fetal ultrasound biometry is recommended. Diagnosis of FGR is based on fetal sonographic measurements, especially AC; estimated fetal weight derived from BPD or HC, AC and FL; and longitudinal progression of fetal growth.

Current evidence suggests the use of a 10th percentile diagnostic threshold for these measurements. Follow the biometric parameters longitudinally, repeating the measurement every 2 to 4 weeks. More frequent examination is unreliable.

2. Fetal surveillanceAntepartum monitoring

Fetuses identified as growth-restricted are at risk for adverse outcomes in utero. Thus, antepartum monitoring is indicated for timely recognition and intervention.

Tests commonly used include the nonstress test (NST), biophysical profile (BPP), sonographic estimation of amniotic fluid volume, and Doppler velocimetry of the umbilical artery or other vessels of interest.

With progressive fetal compromise, compensatory mechanisms may be insufficient to maintain fetal homeostasis. The sequence of loss of compensatory mechanisms is reflected in surveillance tests. Thus, changes in umbil-ical arterial Doppler indices usually precede nonreassuring NST or BPP.

Nonstress test: The standard of care. The NST is probably the most common fetal surveillance test in high-risk pregnancies, including those complicated by FGR. However, it is personnel- and time-intensive, and interpretation often is subjective.

A nonreactive NST is associated with adverse perinatal outcome, although it more commonly is related to the fetal sleep cycle.

In predicting a potentially preventable fetal death, the NST has a false-negative rate of 2 to 3 per 1,000, a negative predictive value of 99.8%, and a false-positive rate of 80%.

Evidence of its efficacy is lacking. Four randomized trials in the early 1980s, which involved more than 1,500 women, found no improvement in outcomes. (All 4 trials lacked sufficient power.)

• When to test. Despite these limitations, the NST has become an integral part of obstetri-cal practice and remains a standard of care for antepartum fetal surveillance in high-risk pregnancies. It should be used as a surveillance tool for the growth-restricted fetus. The test initially is performed weekly.

Depending on severity of growth restriction, testing may increase to twice weekly or even daily.

Amniotic fluid volume monitoring is warranted. Oligohydramnios—highly prevalent in FGR—is associated with adverse perina-tal outcomes.

Although the sonographically determined amniotic fluid index (AFI) is the most frequently used modality to evaluate amniotic fluid volume, its accuracy has been questioned. Nevertheless, a meta-analysis²² of 18 studies involving more than 10,000 patients showed that antepartum AFI equal to or below 5 cm was associated with an increased risk of cesarean section for fetal distress and an Apgar score below 7 at 5 minutes.