The Committee on Practice Bulletins–Obstetrics of the American College of Obstetricians and Gynecologists (ACOG) has developed clinical management guidelines on antepartum fetal surveillance. According to the committee, the goal of antepartum fetal surveillance is to prevent fetal death. The techniques of antepartum fetal surveillance, which are based on the assessment of fetal heart rate patterns, have been in clinical use for nearly 30 years. These guidelines, which replace Technical Bulletin No. 188 issued in January 1994, appear in the October 1999 issue of Obstetrics and Gynecology.
Techniques of Antepartum Fetal Surveillance
Several techniques for antepartum fetal surveillance currently in use are discussed in the ACOG bulletin. These include fetal movement assessment, nonstress test, contraction stress test, fetal biophysical profile, modified biophysical profile and umbilical artery Doppler velocimetry.
FETAL MOVEMENT ASSESSMENT
Fetal movement assessment occurs when the mother perceives a diminution in fetal movement. The mother counts fetal “kicks” as a means of antepartum fetal surveillance. The optimal number of movements and the ideal duration for counting movements have not been determined; however, numerous protocols have been reported and appear to be acceptable.
CONTRACTION STRESS TEST
The contraction stress test is based on the response of the fetal heart rate to uterine contractions. It is believed that fetal oxygenation will be transiently worsened by uterine contractions. In the fetus with suboptimal oxygenation, the resulting intermittent worsening in oxygenation will, in turn, lead to the fetal heart rate pattern of late decelerations. Uterine contractions also may provoke or accentuate a pattern of variable decelerations caused by fetal umbilical cord compression, which in some cases is associated with oligohydramnios.
The contraction stress test is interpreted by the presence or absence of late fetal heart rate decelerations, which are defined as decelerations that reach their nadir after the peak of the contraction and that usually persist beyond the end of the contraction. The results of the contraction stress test are categorized in the ACOG bulletin as follows:
Negative. No late or significant variable decelerations.
Positive. Late decelerations following 50 percent or more of contractions (even if the contraction frequency is fewer than three in 10 minutes).
Equivocal-suspicious. Intermittent late decelerations or significant variable decelerations.
Equivocal-hyperstimulatory. Fetal heart rate decelerations that occur in the presence of contractions that are more frequent than every two minutes or last longer than 90 seconds.
Unsatisfactory. Fewer than three contractions in 10 minutes or a tracing that is not interpretable.
Relative contraindications to the contraction stress test usually include conditions that are associated with an increased risk of preterm labor and delivery, uterine rupture or uterine bleeding. According to ACOG, these conditions include the following:
Preterm labor or certain patients at high risk of preterm labor.
Preterm membrane rupture.
History of extensive uterine surgery or classic cesarean delivery.
Known placenta previa.
In the nonstress test, the heart rate of the fetus that is not acidotic or neurologically depressed will temporarily accelerate with fetal movement. Heart rate reactivity is believed to be a good indicator of normal fetal autonomic function. Loss of reactivity is commonly associated with a fetal sleep cycle but may result from any cause of central nervous system depression, including fetal acidosis.
Results of nonstress tests are classified as reactive or nonreactive. Various definitions of reactivity have been used. Most commonly, the nonstress test is considered reactive, or normal, if there are two or more fetal heart rate accelerations within a 20-minute period, with or without fetal movement discernible by the woman, according to ACOG. The nonreactive stress test lacks sufficient fetal heart rate accelerations over a 40-minute period. The nonstress test of the neurologically healthy preterm fetus is frequently nonreactive—from 24 to 28 weeks of gestation, up to 50 percent of nonstress tests may not be reactive, and from 28 to 32 weeks of gestation, 15 percent of nonstress tests are not reactive.
The biophysical profile discussed in the ACOG bulletin is a nonstress test plus four observations made by real-time ultrasonography. The five components of the biophysical profile are as follows: (1) nonstress test; (2) fetal breathing movements (one or more episodes of rhythmic fetal breathing movements of 30 seconds or more within 30 minutes); (3) fetal movement (three or more discrete body or limb movements within 30 minutes); (4) fetal tone (one or more episodes of extension of a fetal extremity with return to flexion, or opening or closing of a hand; and (5) determination of the amniotic fluid volume (a single vertical pocket of amniotic fluid exceeding 2 cm is considered evidence of adequate amniotic fluid).
Each of the components is given a score of 2 (normal or present as defined previously) or 0 (abnormal, absent or insufficient). A composite score of 8 or 10 is normal, a score of 6 is equivocal and a score of 4 or less is abnormal. In the presence of oligohydramnios, further evaluation is warranted regardless of the composite score.
MODIFIED BIOPHYSICAL PROFILE
During the late second or third trimester, amniotic fluid reflects fetal urine production. Placental dysfunction may cause diminished fetal renal perfusion, which can lead to oligohydramnios. Therefore, assessment of amniotic fluid volume can be used to evaluate long-term uteroplacental function. This led to the development of the modified biophysical profile.
The modified biophysical profile combines the non-stress test with the amniotic fluid index, which is the sum of measurements of the deepest cord-free amniotic fluid pocket in each of the abdominal quadrants, as an indicator of long-term function of the placenta. An amniotic fluid index of more than 5 cm is thought to be an adequate volume of amniotic fluid. The modified biophysical profile is considered normal if the nonstress test is reactive and the amniotic fluid index is greater than 5 cm and abnormal if the nonstress test is nonreactive or the amniotic fluid index is 5 cm or less.
UMBILICAL ARTERY DOPPLER VELOCIMETRY
Doppler ultrasonography is used to assess the hemodynamic components of vascular impedence. Umbilical artery Doppler flow velocimetry has been adapted as a fetal surveillance technique because it is believed that flow velocity waveforms in the umbilical artery of fetuses with normal growth differ from those of fetuses with growth restriction. The umbilical flow velocity waveform of a normally growing fetus has high-velocity diastolic flow, while in cases of intrauterine growth restriction, the umbilical artery diastolic flow is diminished. With extreme intrauterine growth restriction, the flow may be absent or even reversed. There is a high perinatal mortality rate among such pregnancies.
Indications for Antepartum Fetal Surveillance
The results of antepartum fetal surveillance have not definitively demonstrated improved perinatal outcome. Therefore, all indications for antepartum testing should be considered somewhat relative. Usually, antepartum fetal surveillance is used in pregnancies with a high risk of antepartum fetal death. Some of the conditions in which testing is appropriate include the following:
Maternal conditions: antiphospholipid syndrome, poorly controlled hyperthyroidism, hemoglobinopathies such as hemoglobin SS, SC or S-thalassemia, cyanotic heart disease, systemic lupus erythematosus, chronic renal disease, type 1 diabetes mellitus and hypertensive disorders.
Pregnancy-related conditions: pregnancy-induced hypertension, decreased fetal movement, oligohydramnios, polyhydramnios, intrauterine growth restriction, post-term pregnancy, moderate to severe isoimmunization, previous fetal demise (unexplained or recurrent risk) and multiple gestation with significant growth discrepancy.
The following ACOG recommendations are based on limited or inconsistent scientific evidence (Level B):
Women at high risk for stillbirth should undergo antepartum fetal surveillance using the nonstress test, contraction stress test, biophysical profile or modified biophysical profile.
Initiation of testing at 32 to 34 weeks of gestation is appropriate for most pregnancies that are at increased risk of stillbirth. In pregnancies with multiple or particularly worrisome high-risk conditions, testing may be initiated as early as 26 to 28 weeks of gestation.
When the clinical condition that prompted testing persists, a reassuring test should be repeated weekly or, depending on the test used and the presence of certain high-risk conditions, twice weekly until delivery. Any significant deterioration in fetal activity requires fetal reevaluation, regardless of the amount of time that has elapsed since the last test.
An abnormal nonstress test or modified biophysical profile usually should be further evaluated by a contraction stress test or a full biophysical profile. Subsequent management should then be predicated on the results of the contraction stress test or biophysical profile, the gestational age, the degree of oligohydramnios (if assessed) and the maternal condition.
Oligohydramnios, defined as no ultrasonographically measurable vertical pocket of amniotic fluid greater than 2 cm or an amniotic fluid index of 5 cm or less, requires (depending on the degree of oligohydramnios, the gestational age and the maternal clinical condition) delivery, or close maternal or fetal surveillance.
In the absence of obstetric contraindications, delivery of the fetus with an abnormal test result often may be attempted by induction of labor with continuous monitoring of the fetal heart rate and contractions. If repetitive late decelerations are observed, cesarean delivery generally is indicated.
Recent, normal antepartum fetal test results should not preclude the use of intrapartum fetal monitoring.
Umbilical artery Doppler velocimetry seems to benefit only pregnancies complicated by intrauterine growth restriction. If used in this setting, decisions regarding timing of delivery should be made using a combination of information from the Doppler ultrasonography and other tests of fetal well-being, along with careful monitoring of maternal status.
Middle cerebral artery Doppler velocimetry should be considered an investigational approach to antepartum fetal surveillance.