A fetal stress test, also known as a contraction stress test (CST), evaluates a baby’s heart rate response to uterine contractions. It determines if the baby can handle the reduced oxygen supply that occurs during labor. This assessment helps healthcare providers understand the baby’s well-being and potential tolerance for the birthing process.
Reasons for a Fetal Stress Test
A fetal stress test is recommended when concerns arise about a baby’s health or oxygen supply during pregnancy. It is often performed as a follow-up if a nonstress test (NST) or a biophysical profile (BPP) indicates a potential problem. These prior tests might suggest issues warranting a more in-depth assessment of how the baby responds to contractions.
The test is also used in high-risk pregnancies with pre-existing conditions or complications that could affect fetal well-being. Conditions such as diabetes, preeclampsia, or suspected placental insufficiency can lead to concerns about the baby’s ability to receive adequate oxygen during labor. A CST provides valuable information to guide management decisions.
Performing the Test
To begin a fetal stress test, the mother lies on her back with the head slightly elevated. Two belts with sensors are placed around the abdomen. One sensor monitors the baby’s heart rate, while the other records uterine contractions. This allows observation of the relationship between contractions and the baby’s heart rate.
Contractions are stimulated to mimic those experienced during labor. This is achieved through two methods: nipple stimulation or intravenous (IV) administration of oxytocin (Pitocin). Nipple stimulation causes the body to release natural oxytocin, prompting contractions. If nipple stimulation does not produce sufficient contractions, synthetic oxytocin is given through an IV line, with the dosage gradually increased until adequate contractions are achieved. The goal is to produce at least three contractions lasting 40 to 60 seconds each within a 10-minute period.
The test typically lasts up to two hours. While contractions are generally mild and may feel like menstrual cramps, the process carries a small risk of inducing labor, particularly if contractions become too frequent or strong. The test is usually not performed if there are contraindications such as a history of premature birth or certain uterine conditions.
Understanding Test Results
A “negative” or normal result is the desired outcome. This means the baby’s heart rate remains stable, showing no significant slowing (late decelerations) after more than half of the contractions. A negative result suggests that the baby is likely well-oxygenated and capable of tolerating the demands of labor.
Conversely, a “positive” or abnormal result indicates that the baby’s heart rate consistently slows down after contractions. This pattern, known as late decelerations, suggests the baby may not be receiving enough oxygen from the placenta. A positive result may prompt further evaluation, such as additional tests or consideration of an expedited delivery, potentially by cesarean section. Sometimes, results can be “equivocal” or “suspicious,” showing intermittent decelerations, which often necessitates repeat testing or closer monitoring.
Alternative Fetal Health Assessments
Other antenatal surveillance methods assess fetal well-being. The nonstress test (NST) is a common non-invasive procedure that monitors the fetal heart rate in response to fetal movement. A “reactive” NST indicates a healthy baby, showing accelerations in heart rate with movement.
Another assessment is the biophysical profile (BPP), which combines an NST with an ultrasound examination. The BPP evaluates five specific parameters:
- Fetal breathing movements
- Fetal body movements
- Fetal tone
- Amniotic fluid volume
- The nonstress test result
Each parameter is scored, providing an overall picture of fetal health.
Umbilical artery Doppler velocimetry is a specialized ultrasound technique that measures blood flow in the umbilical artery, providing insights into placental function and resistance. Abnormal flow patterns can indicate issues like placental insufficiency or fetal growth restriction. These tests offer different perspectives on fetal health, helping healthcare providers make informed decisions about pregnancy management.