Antepartum fetal surveillance: Clinical sciences

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Introduction0:00–0:52

Antepartum fetal surveillance is the evaluation of fetal well-being, which is performed after 32 weeks of gestation to reduce the risk of stillbirth.
In the third trimester, the fetal heart rate pattern, fetal activity, and the amniotic fluid volume are sensitive to fetal oxygen levels and acid-base status.
Surveillance techniques, such as fetal heart rate tracing and real-time ultrasound, can identify a compromised fetus and provide an opportunity to intervene before worsening hypoxemia and metabolic acidosis result in fetal demise.
However, these tests don’t reflect the severity or duration of oxygenation and acid-base impairment, and don’t predict stillbirths related to acute events, such as placental abruption or an umbilical cord prolapse.

Focused history0:52–2:47

The first step in evaluating a patient who presents for antepartum fetal surveillance is to obtain a focused history. You’ll start by identifying the indication for testing, which will help you determine the preferred surveillance method and testing intervals.
A number of maternal, fetal, and placental factors are associated with an increased risk of stillbirth, and multiple risk factors can add up.
Monitoring fundal height measurements during prenatal care visits is one of the first steps in determining fetal well-being.
After 20 weeks of gestation, the fundal height should correlate to estimated gestational age. A discrepancy in fundal height should prompt you to obtain a growth scan.
Certain maternal conditions typically prompt serial growth ultrasounds starting in the second trimester, including hypertension or preeclampsia; pre-existing diabetes; obesity; a history of previous pregnancy with growth restriction; and multifetal gestation.
Fetal growth is an important factor to test, because fetal growth restriction is strongly associated with an increased risk of stillbirth, as well as other perinatal complications, including perinatal asphyxia, neurodevelopmental impairment, and complications related to prematurity, such as respiratory distress syndrome.
Finally, knowing the gestational age at the time of testing is also important, because test interpretation and management options can vary based on fetal gestational age.Here's a high yield fact!
Fetal growth restriction is defined as either an estimated fetal weight or fetal abdominal circumference that’s less than the 10th percentile for gestational age.
Severe fetal growth restriction is present when either measurement is less than the 3rd percentile. Speaking of, let’s talk about normal fetal growth.

Normal fetal growth2:47–11:05

Your next step is to perform a nonstress test, or NST, and depending on the indication for testing, you might also check the amniotic fluid volume, or AFV.
When the amniotic fluid is measured by ultrasound at the time of an NST, this combination is called a modified biophysical profile or BPP.
Now, the idea behind the NST is that fetuses that are not neurologically depressed or acidotic will briefly accelerate their heart rate above baseline when they move.
The test is performed by placing an external Doppler ultrasound transducer on the maternal abdomen to detect and record the fetal heart rate for at least 20 minutes.
The recording is displayed on a graph or monitor, allowing interpretation of the fetal heart rate pattern in real time. External monitoring of uterine contractions is performed simultaneously with a tocotransducer, which measures the change in pressure on the maternal abdomen as the uterus contracts.
Contraction frequency and duration are then recorded on the graph.Once the patient is correctly positioned and the external monitors are applied, you’ll want to evaluate the fetal heart rate baseline; variability, which are the normal fluctuations in the fetal heart rate; and accelerations; as well as any decelerations, or decreases in the fetal heart rate below baseline.
If the amniotic fluid volume is measured for a modified BPP, a normal fluid volume is defined as the deepest vertical pocket of fluid measuring greater than 2 cm.
Oligohydramnios, or low amniotic fluid, is present when the deepest vertical pocket is 2 cm or less. Here’s a clinical pearl!
Evaluating the amniotic fluid volume in a patient without ruptured membranes provides valuable information about uteroplacental function.
Because amniotic fluid represents fetal urine production, placental dysfunction that leads to fetal hypoxemia may result in diminished kidney perfusion due to redistribution of blood flow.
This process can lead to reduced fetal urine production and oligohydramnios. Alright, your next step is to determine if the fetal heart rate tracing reflects any pattern that indicates an immediate risk of a compromised fetus.
If you see any of these: an absence of variability; late or prolonged decelerations; repetitive variable decelerations; bradycardia; or a sinusoidal pattern that’s associated with fetal anemia, hypoxia, or acidosis; you need to act right away!
Based on the fetal heart rate pattern, treatment includes intrauterine resuscitation, meaning IV fluid replacement and maternal repositioning on their left side to improve uterine blood flow, hospitalization, additional monitoring like a BPP for a fetus less than 37 weeks, and consideration for delivery, especially if the fetus is 37 weeks or greater.
Fortunately, most patients who present for antepartum fetal surveillance won’t require these emergency measures and can proceed with the standard NST.
Okay, let’s go one step back. NST results are categorized as reactive, which is normal; or nonreactive, which is inconclusive and requires additional evaluation.
A reactive NST is defined by the presence of two or more fetal heart rate accelerations within a 20-minute period. Importantly, the criteria for fetal heart rate accelerations on an NST vary based on gestational age.
For a fetus at 32 weeks or greater, each of the two required fetal heart rate accelerations must peak at least 15 beats above the baseline and last for at least 15 seconds.
However, a fetus between 28 and 32 weeks may not be mature enough to meet the standard criteria for a reactive NST, so a lower threshold is used.
Moreover, because prematurity makes antepartum tests difficult to interpret, they’re not performed earlier than 28 weeks.
On the other hand, a nonreactive NST is when there are insufficient accelerations over a 40 minute period. Because the most common cause of a nonreactive NST is the fetal sleep cycle, you can safely extend an NST to 40 minutes if needed to detect reactivity.
In addition, you may consider vibroacoustic stimulation to elicit a fetal response. To do this, place the device on the maternal abdomen and apply a stimulus for 1 to 2 seconds.
If you don't get a response, you can repeat it up to three times, for progressively longer durations of up to 3 seconds.Alright, so if the NST is reactive, and if the amniotic fluid volume, if measured, is normal, the fetus is at low risk for stillbirth in the next 7 days.
You can schedule repeat testing at recommended intervals. On the other hand, if your patient has a nonreactive NST, or if oligohydramnios is present, the next step is to perform a complete BPP.
The BPP has five components, including the results of the NST and four additional observations made during a 30-minute ultrasound exam.
During the ultrasound, look for at least one episode of fetal breathing movement that lasts at least 30 seconds; three or more body or limb movements; fetal tone demonstrated by an episode of extension of an extremity with return to flexion, such as the opening and closing of a hand; and check for a normal amniotic fluid volume if it hasn’t been determined yet.
Each of these 5 components is assigned a score of either zero if not present or 2 if it is present. A composite score of 8 or 10 out of 10 is normal, and you can schedule these patients for retesting at recommended intervals.
Next, a score of 6 out of 10 is equivocal and needs further testing, such as a repeat BPP in 24 hours, or delivery if the fetus is 37 weeks or greater.
A score that’s 4 out of 10 or lower is abnormal and usually warrants delivery. However, you should individualize management if the fetus is less than 32 weeks.
Lastly, regardless of the score, if there’s isolated oligohydramnios without ruptured membranes, they’ll need individualized management; but if the fetus is 36 weeks or greater, consider delivery.One more clinical pearl!
Another lesser-used option for antepartum fetal surveillance is a contraction stress test or CST. This evaluates the fetal heart rate in response to uterine contractions, which cause a temporary reduction in blood flow to the intervillous space and decreased oxygenation to the fetus.
If the fetus has reduced oxygen reserves from placental insufficiency, the intermittent decrease in oxygenation during a contraction can lead to a fetal heart rate pattern of late decelerations.
Contractions can also produce a pattern of variable decelerations caused by umbilical cord compression, especially if oligohydramnios is present.
A negative, or normal, CST shouldn’t have late or variable decelerations; whereas, a positive, or abnormal, CST occurs when at least 50% of the contractions are associated with late decelerations.
If the CST is positive, either proceed with a BPP or consider delivery.Now let’s go all the way back and talk about pregnancies presenting with fetal growth restriction.

Fetal growth restriction11:05–12:21

A fetus with growth restriction is at high risk for stillbirth, and should undergo antepartum fetal surveillance with regular BPPs and umbilical artery Doppler velocimetry.
Umbilical artery Doppler velocimetry is a noninvasive ultrasound technique that assesses changes in umbilical artery blood flow.
On Doppler testing, a normally growing fetus demonstrates high-velocity diastolic flow in the umbilical cord. In contrast, a growth-restricted fetus may have decreased and sometimes even absent or reversed diastolic umbilical artery flow.
The mortality rate for these fetuses is significantly increased. Now, if BPP is abnormal, you’ll proceed the same as before.
On the other hand, if both Doppler studies and the BPP are normal, schedule the patient for additional testing at recommended intervals.
However, if Doppler testing is abnormal, even if BPP is normal, the patient should undergo more frequent surveillance and possibly hospitalization with consideration for delivery.

Review12:21–12:57

Alright, as a quick recap… Antepartum fetal surveillance is performed in the third trimester to reduce the risk of stillbirth and can be conducted several different ways.
The most common initial tests include an NST or a modified biophysical profile. Because most fetuses with an abnormal initial test are actually healthy, you should consider a secondary test, such as a BPP, before deciding on appropriate management.
If a fetus is affected by growth restriction, the recommended testing is a BPP with umbilical artery Doppler