A commentary on ACOG Practice Bulletin Number 145 by Contemporary OB/GYN editorial board member Haywood L. Brown, MD.
Committee on Practice Bulletins-Obstetrics
ACOG Practice Bulletin Number 145: Antepartum Fetal Surveillance, July 2014. Obstet Gynecol. 2014; 124:182–92. Full text of ACOG Practice Bulletins is available to ACOG members at http://www.acog.org/Resources-And-Publications/Practice-Bulletins/Committee-on-Practice-Bulletins-Obstetrics/Antepartum-Fetal-Surveillance
Antepartum Fetal Surveillance
The goal of antepartum fetal surveillance is to prevent fetal death. Antepartum fetal surveillance techniques based on assessment of fetal heart rate (FHR) patterns have been in clinical use for almost four decades and are used along with real-time ultrasonography and umbilical artery Doppler velocimetry to evaluate fetal well-being. Antepartum fetal surveillance techniques are routinely used to assess the risk of fetal death in pregnancies complicated by preexisting maternal conditions (eg, diabetes mellitus) as well as those in which complications have developed (eg, fetal growth restriction). The purpose of this document is to provide a review of the current indications for and techniques of antepartum fetal surveillance and outline management guidelines for antepartum fetal surveillance that are consistent with the best scientific evidence.
Used with permission. Copyright the American College of Obstetricians and Gynecologists.
By Haywood L. Brown, MD
Dr. Brown is F. Bayard Carter Professor and Chair, Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Duke University School of Medicine, Durham, North Carolina.
For nearly 4 decades, fetal heart monitoring (FHR) has been used to assess antenatal and intrapartum fetal well-being. While both antenatal and intrapartum monitoring have come under criticism, antepartum fetal heart rate surveillance to assess the risk of fetal death and stillbirth is less controversial for the purpose for which it was intended when it was introduced in the 1970s.
This Practice Bulletin provides a review of the indications and techniques for antepartum fetal surveillance with FHR being the consistent parameter used in the assessment of fetal well-being.
Abnormal fetal surveillance is based on physiologic changes that alter fetal heart rate and fetal activity. Fetal heart rate, fetal movement, and tone in particular are impacted by uteroplacental fetal blood flow alterations and are thereby sensitive to fetal hypoxemia and acidemia. While nonreassuring fetal surveillance is associated with fetal hypoxemia and acidemia based on these physiologic adjustments, these indicators can neither predict the degree or duration of the fetal acid base disturbance nor precisely predict neonatal outcome.
A warning sign that a fetus may be at risk of compromise is maternal perception of decrease in fetal movement. If “kick counting” is used by the patient, a nonreassuring count provides the alert for further assessment. Many approaches to counting kicks have been used over the past decades, but the perception of 10 distinct movements in a period of up to 2 continuous or interrupted hours is considered reassuring. A nonreassuring count should prompt notification for further fetal assessment.
The non-stress test (NST) and the ultrasound biophysical profile (BPP) are the primary antenatal fetal surveillance methods now used. The NST is based on the principle that the fetal heart will accelerate with movement in a fetus with normal autonomic function. Accelerations of 15 beats per minute above baseline and for 15 seconds from the baseline in a 20- to 40-minute period are considered reactive and are a measure that has stood the test of time as a predictor of fetal well-being at that point in time.
A normal BPP score along with a reactive NST is an indication of fetal well-being. The BPP provides 2 points each for fetal breathing, movement, and fetal tone in 30 minutes and 2 points for normal amniotic fluid volume. There has been debate regarding the ultrasound definition of oligohydramnios and whether a single deepest vertical pocket of fluid of ≤2 cm, as recommended in the Practice Bulletin, is more acceptable as a predictor than an amniotic fluid index (AFI) of <5 cm. Oligohydramnios suggests renal under-perfusion and decreased fetal urination and should prompt further evaluation or delivery, especially if other biophysical parameters are altered. A total biophysical score of <4 is abnormal and suggestive of fetal compromise and increased risk of adverse outcome.
With the addition of umbilical artery Doppler velocimetry, particularly in the surveillance of fetal growth restriction (FGR), the contraction stress test (CST) is now rarely used to assess for fetal compromise or potential hypoxemia. There is no evidence that inclusion of umbilical artery Doppler interrogation in antenatal surveillance provides additional benefit in the assessment of a normally growing fetus.
Current clinical practice calls for performing a NST once or twice weekly after 32 0/7 gestational weeks, depending on the indication for testing. If the indication for testing is not persistent, the NST need not be repeated. If the maternal condition is stable and testing is reassuring, the NST is typically repeated weekly.
The negative predictive value of NST alone for predicting stillbirth within 1 week of a normal test is 99.8%; for BPP, modified BPP, and CST, it is greater than 99.9%.
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Antenatal testing is used for pregnancies considered at risk of antepartum stillbirth, such as those complicated by pre-gestational diabetes, poorly controlled gestational diabetes, maternal vascular disease (chronic hypertension), and FGR. (See Box 1 of the Practice Bulletin.)2
Initiation of testing at 32 0/7 weeks is appropriate for most women at risk with the exception of patients with FGR recognized prior to 32 weeks’ gestation. The challenge for the clinician is acting on an abnormal (false-positive) test result, which has the potential for iatrogenic premature delivery with resultant complications of prematurity. In one large single-center review, 60% of fetuses with an abnormal antepartum test result had no evidence of short-term or long-term fetal compromise.1 While statistically these are “false positives,” the clinician has to take into consideration the clinical indications for testing in the first place and the goal of the test in preventing stillbirth.
Management depends on gestational age, maternal condition, and which antenatal testing or combination thereof is abnormal. Abnormal results of an NST (nonreactive) should be followed by a BPP, modified BPP, or a CST.
A BPP of 6 is considered equivocal and prompts consideration for delivery, especially beyond 37 0/7 weeks, or repeat testing in 24 hours if <37 weeks. A BPP score <4 is an indication for delivery in most circumstances with a viable fetus. If the gestational age is <32 0/7 weeks, maternal steroid administration and extended monitoring is appropriate but such management should be individualized, especially in cases of FGR. The FGR guidelines from the Society for Maternal-Fetal Medicine suggest delivery if there is absent end-diastolic flow at or beyond 34 0/7 weeks and with reverse end diastolic flow delivery if gestational age is ≥32 0/7 weeks.3
Management decisions are more challenging in cases with oligohydramnios as the only abnormality in antenatal surveillance. Most experts agree that the best course of action with a finding of isolated and uncomplicated oligohydramnios is expectant management if gestational age is <36 0/7 weeks and delivery if gestational age is >36 completed weeks.
Antenatal fetal surveillance has stood the test of time with regard to the goal of preventing stillbirth in the fetus at risk based on indications for testing. As such, the additional cost for testing in the appropriate setting appears to have benefit. However, clinicians should be reminded that the least costly antenatal surveillance modality is maternal fetal movement assessment as a test for well-being in low- and high-risk women, even if its effectiveness in preventing stillbirth is uncertain.
Fetal kick counting in current antenatal clinical care appears to be underutilized and clinicians should be reminded to educate women about this modality in antenatal care.
1. Miller DA, Rabelllo YA, Paul RH. The modified biophysical profile: Antepartum testing in the 1990s. Am J Obstet Gynecol. 1996;174:812–817.
2. American College of Obstetricians and Gynecologists Practice Bulletin number 145: Antenatal Fetal Surveillance. Obstet Gynecol. 2014;124:182–192.
3. Doppler assessment of the fetus with intrauterine growth restriction. Society for Maternal-Fetal Medicine. Am J Obstet Gynecol. 2012;206:300–308.