Antepartum fetal surveillance: Clinical sciences

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.

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. 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.