The Non-Stress Test (NST) is a simple, non-invasive procedure used during pregnancy to monitor the well-being of the fetus. This assessment gauges the fetal heart rate’s response to movement, which helps providers ensure the fetus is receiving adequate oxygen supply. The name comes from the fact that it places no physical stress on the pregnant person or the fetus. While the standard recording period is short, several factors can influence the total duration of the appointment.
When and Why the Non-Stress Test is Ordered
The Non-Stress Test is generally performed in the third trimester, typically after 28 weeks of gestation, when the fetal nervous system is mature enough to respond to movement with a heart rate change. Healthcare providers order this test when there is a need for closer monitoring of the fetus’s environment. This need often arises in pregnancies considered high-risk due to chronic maternal conditions.
Conditions such as gestational diabetes, preeclampsia, or chronic high blood pressure are common indications for regular monitoring. The NST is also frequently ordered if the pregnancy goes past the due date, beyond 40 weeks, or if there is any suspicion that the placenta may not be functioning optimally. A significant reason for ordering an immediate NST is a maternal report of decreased fetal movement, as this can be an early sign that the fetus is not getting enough resources. Other factors, including carrying multiples or having a history of pregnancy complications, also make this regular screening necessary.
The Standard Procedure and Expected Duration
The technical goal of the Non-Stress Test is to record the fetal heart rate and movement over a defined period. The pregnant person is typically placed in a reclining chair or on an examination table, often tilted slightly to the left to maximize blood flow. Two elastic belts are then secured around the abdomen: one contains a sensor to track the fetal heart rate, and the other monitors for uterine contractions.
The standard monitoring period for the NST is 20 minutes. Within this timeframe, the technician is looking for the fetal heart rate to accelerate, or increase, at least two times in response to movement. For pregnancies at or after 32 weeks, an acceleration is defined as an increase of at least 15 beats per minute above the baseline rate, lasting for a minimum of 15 seconds. This specific two-acceleration criterion within 20 minutes is the benchmark for a successful and timely test completion.
Reasons Testing Time May Be Extended
Although the standard monitoring time is 20 minutes, the total time spent in the testing room often extends to 30 to 40 minutes, and sometimes longer. The most frequent reason for a time extension is the fetal sleep cycle. The fetus may be in a natural sleep state, which typically lasts around 20 to 40 minutes, during the initial monitoring period.
If the fetus remains inactive and the acceleration criteria are not met, the healthcare provider will attempt to stimulate movement to wake the fetus. One common intervention is the use of a fetal acoustic stimulator, a device placed on the abdomen that emits a gentle sound and vibration, similar to a soft buzzer. Other non-invasive strategies include encouraging the mother to change position, drink cold water, or have a small snack. These interventions can extend the test duration to 40 minutes or more as the monitoring continues until the required heart rate data is successfully captured.
Understanding the Test Results and Next Steps
The outcome of the Non-Stress Test is categorized into two main results: “reactive” or “non-reactive.” A test is considered “reactive” when the two required heart rate accelerations within the 20-minute period are successfully recorded, signaling adequate oxygenation and a healthy central nervous system response. A reactive result is reassuring and typically indicates that the fetus is doing well and does not require immediate further testing.
A “non-reactive” result means that the fetus did not meet the criteria of two accelerations within the initial monitoring period, even after attempts to stimulate movement. While this result can be concerning, it does not automatically mean there is a problem; it often simply means the fetus was too sleepy or the test was inconclusive. If the result remains non-reactive after extended monitoring, further testing is necessary to gain a clearer picture of fetal health. The most common next step is a Biophysical Profile (BPP), which combines another brief period of monitoring with an ultrasound to assess other fetal parameters like movement, breathing, and amniotic fluid volume.