The Non-Stress Test (NST) is a common, non-invasive prenatal procedure used to monitor fetal well-being in the later stages of pregnancy. This simple test observes the baby’s heart rate in response to their movements. The primary goal is to determine if the fetus is receiving adequate oxygen and demonstrating appropriate responsiveness. A healthy baby’s heart rate temporarily increases when they move, which signals that the baby’s central nervous system is functioning normally.
When is the Test Performed
A healthcare provider typically orders an NST when closer monitoring of fetal health is needed, usually starting after 28 weeks of gestation. This timing is selected because the fetal nervous system is mature enough to reliably show the expected heart rate changes. The test is frequently initiated for high-risk pregnancies due to pre-existing maternal conditions like chronic high blood pressure, diabetes, or kidney disease.
The NST is also common when a pregnancy goes past the expected due date, as placental function can decline after 40 weeks. A reported decrease in the frequency or strength of fetal movement is another common indication. If there are concerns about the placenta’s ability to deliver sufficient nutrients and oxygen, an NST provides a fast assessment of the fetus’s current condition.
The test may also be necessary for concerns about the baby’s growth, such as being small for gestational age, or complications from carrying multiple babies. Regular NSTs are also used for women with Rh sensitization, where maternal antibodies could affect the baby’s red blood cells. The goal is to screen for potential issues with oxygen supply before they become significant problems.
Step-by-Step Procedure
The Non-Stress Test is performed while the mother lies comfortably in a semi-reclined position, often tilted slightly to the side to ensure proper blood flow. The procedure involves securing two elastic belts around the abdomen, each holding a specialized sensor called a transducer. This setup allows for continuous external monitoring.
One transducer uses Doppler technology to record the fetal heart rate, while the second sensor, a tocodynamometer, monitors uterine contractions and fetal movement. The heart rate and any detected contractions are printed on a tracing or displayed on a digital monitor. The procedure typically lasts about 20 minutes, but may be extended to 40 minutes if the baby is asleep or inactive.
The mother is given a handheld button to press every time she feels the baby move. This action places a mark on the tracing strip, allowing the clinician to correlate fetal movement with heart rate changes. If the baby remains quiet, a clinician may use a gentle noise-making device, called a vibroacoustic stimulator, placed on the abdomen to encourage activity.
Understanding Your Results
The outcome of the Non-Stress Test is categorized as either “Reactive” or “Non-Reactive,” based on the heart rate patterns recorded. A “Reactive” result is the desired outcome, signifying that the baby’s nervous system is appropriately regulating heart function. For a fetus at 32 weeks gestation or older, a reactive result requires specific criteria to be met within the 20-minute monitoring period.
The tracing must show at least two distinct accelerations of the fetal heart rate. Each acceleration needs to reach a peak at least 15 beats per minute (BPM) above the baseline heart rate. Furthermore, each acceleration must last for a minimum of 15 seconds before the rate returns to the baseline.
A “Non-Reactive” result occurs when the test fails to meet these acceleration criteria within the standard monitoring time. This outcome does not mean the baby is in distress, but it indicates the test was not reassuring. The most common reason for a non-reactive tracing is that the baby was in a quiet sleep cycle, which typically lasts between 20 and 40 minutes.
A non-reactive result can also signal that the baby is not receiving adequate oxygen or that the central nervous system is depressed. The lack of heart rate acceleration with movement suggests the baby’s physiological response to activity is compromised. A prolonged non-reactive tracing requires immediate further evaluation to determine the true cause.
Safety and Next Steps
The non-stress test is considered very safe for both the mother and the baby because it is non-invasive and uses only external monitoring devices. There are no known risks associated with the procedure, as it does not involve needles, radiation, or medications. The test records the baby’s natural physiological responses to assess current health status.
If the initial 20 to 40 minutes of monitoring yields a non-reactive result, the provider may extend the test for another 20 minutes, sometimes using a vibroacoustic stimulator to encourage activity. If the result remains non-reactive after this extended period, further testing is required to clarify the baby’s condition.
The most common follow-up is a Biophysical Profile (BPP), which combines the NST with a comprehensive ultrasound examination. The BPP evaluates several factors:
- The baby’s breathing movements
- Body movements
- Muscle tone
- The volume of amniotic fluid
Alternatively, a Contraction Stress Test (CST) may be performed to see how the baby’s heart rate reacts to temporary, mild uterine contractions.
For those with high-risk pregnancies, the Non-Stress Test is often performed regularly, sometimes once or twice a week, throughout the third trimester. The choice of the next step is determined by the provider based on the results, the baby’s gestational age, and the overall clinical picture.