Antepartum fetal surveillance refers to the practice of monitoring a baby’s well-being before labor begins, particularly in pregnancies identified as having a higher risk of complications. These evaluations use non-invasive tools to assess the health of the fetus and the function of the placenta. While patients often use the broad term “fetal stress test,” this typically refers to a specific set of procedures designed to check how the baby responds to its internal environment. Monitoring the baby’s condition through these tests helps healthcare providers make informed decisions about the timing and method of delivery.
Defining the Non-Stress Test (NST)
The Non-Stress Test (NST) is the most frequently performed screening tool in antepartum surveillance and is often what people mean when they mention a fetal stress test. This test is considered non-invasive because it involves no internal procedures and places no physical demand on the mother or the fetus. The procedure typically takes between 20 to 40 minutes to complete, allowing time to observe the baby’s natural activity cycles.
The test is performed with the mother lying in a semi-reclined position, with two external monitors secured to the abdomen using elastic belts. One device, a tocodynamometer, detects uterine activity, while the other, a Doppler ultrasound transducer, tracks the fetal heart rate (FHR). The core purpose of the NST is to record the fetal heart rate in response to the baby’s spontaneous movements.
A healthy fetus should demonstrate a transient increase in heart rate when it moves, indicating a well-oxygenated state and a properly functioning central nervous system. If the baby is inactive during the initial 20 minutes, which often happens during a fetal sleep cycle, the monitoring may be extended or a gentle sound stimulus may be used to wake the fetus.
Interpreting Non-Stress Test Results
The results of the Non-Stress Test are categorized as either “reactive” or “non-reactive,” with a reactive result indicating a normal and reassuring outcome. A reactive test demonstrates that the fetal heart rate is accelerating appropriately in response to movement. Specifically, a healthy fetus at or after 32 weeks of gestation must show two or more heart rate accelerations within a 20-minute monitoring period.
An acceleration is a temporary increase in the fetal heart rate that peaks at least 15 beats per minute (bpm) above the baseline rate and lasts for a minimum of 15 seconds before returning to the baseline. For gestations earlier than 32 weeks, the threshold for a reactive test is slightly lower, requiring accelerations of at least 10 bpm above baseline for 10 seconds. The presence of these accelerations confirms that the baby has adequate oxygen reserves and a healthy neurological response.
A non-reactive result is recorded when the tracing does not meet the specified criteria for accelerations within the initial 20-minute window, even after a total of 40 minutes of observation. This outcome does not automatically signal a problem, as it can frequently be attributed to a fetal sleep state, which typically lasts 20 to 40 minutes. Certain maternal medications, like sedatives, can also temporarily suppress fetal heart rate reactivity, leading to a non-reactive reading.
When a test is non-reactive, it means more information is required to assess the baby’s well-being. Clinicians will then typically pursue further, more comprehensive testing, such as a biophysical profile or a Contraction Stress Test. The goal of these follow-up procedures is to distinguish between a healthy but temporarily quiet fetus and one that may be experiencing a lack of sufficient oxygen supply from the placenta.
The Contraction Stress Test (CST)
The Contraction Stress Test (CST) is a follow-up procedure often used when a Non-Stress Test yields a non-reactive result. This test is designed to simulate the temporary reduction in blood flow and oxygen supply to the baby that occurs during a uterine contraction. The CST helps determine the baby’s ability to tolerate the reduced oxygen levels that would be experienced during labor.
To perform the CST, mild contractions are induced in a controlled environment. This can be achieved through two primary methods: either by having the mother gently massage her nipples, which stimulates the natural release of oxytocin, or by administering a low-dose intravenous infusion of synthetic oxytocin (Pitocin). The goal is to achieve three contractions lasting at least 40 seconds each within a 10-minute period.
The baby’s heart rate is continuously monitored during these contractions, with the focus on identifying any pattern of heart rate slowing, known as decelerations. A “negative” CST result suggests the fetus is healthy enough to withstand the stresses of labor. A negative result means there were no late decelerations—a persistent drop in heart rate that begins after the contraction has peaked and recovers only after the contraction has ended.
Conversely, a “positive” CST result is concerning and indicates that the fetal heart rate showed consistent late decelerations with more than half of the contractions. This pattern suggests that the baby may not have the necessary oxygen reserves to tolerate the physical demands of labor. A positive result often leads to a recommendation for more immediate intervention or delivery, depending on the baby’s gestational age.
When Fetal Monitoring Is Necessary
Fetal surveillance tests are not routinely performed in all pregnancies but are reserved for situations where there is an increased risk of complications. These evaluations serve as a preventative measure to monitor the baby’s condition when risk factors suggest placental function could be compromised. Conditions affecting the mother are common reasons for ordering regular surveillance, such as pre-existing diabetes or gestational diabetes that is not well-controlled.
Maternal hypertension, including chronic high blood pressure or pre-eclampsia, also prompts regular monitoring due to the potential impact on placental blood flow. Other indications include the suspicion of intrauterine growth restriction (IUGR), where the baby is measuring smaller than expected. A significant decrease in the mother’s perception of fetal movement is an immediate reason for a test to be ordered.
Monitoring is also typically initiated for pregnancies that extend past the due date (post-term pregnancies) or for women who have had a previous pregnancy loss, such as a stillbirth. The regular use of tests like the NST or CST helps clinicians track the baby’s health over time. This consistent assessment ensures that any developing problems are identified early, allowing for timely medical intervention.