A Non-Stress Test (NST) is a common, non-invasive prenatal assessment used to monitor the well-being of a developing baby, typically during the third trimester. The test observes the baby’s heart rate and how it responds to movement, which helps health care providers ensure the baby is receiving sufficient oxygen. This procedure is named “non-stress” because it places no physical stress on the mother or the fetus, relying on the baby’s natural activity patterns. The results provide valuable information about the baby’s condition.
Why Healthcare Providers Order NSTs
Health care providers order an NST to confirm the baby’s health, often in pregnancies with certain risk factors. It is frequently recommended for individuals with chronic medical conditions, such as diabetes, hypertension, or heart disease, which can affect placental function and oxygen delivery. The test is also routine for monitoring pregnancies that extend past the due date, typically beyond 40 weeks.
The NST is also ordered if the mother reports a decrease in the baby’s movement, which could signal a change in condition. Additionally, the test is used in cases of multiple gestations (twins or triplets) or if there are concerns about the baby’s growth, such as being small for gestational age. The NST is performed after 28 weeks of gestation, when the fetal nervous system is mature enough for the heart rate to consistently react to movement.
The Typical Duration of the Non-Stress Test
The actual monitoring time for an NST is generally brief, typically lasting between 20 and 40 minutes. The goal is to observe the baby’s heart rate over a 20-minute window to see if it meets the criteria for a healthy, or “reactive,” result.
The test may be extended up to 40 minutes if the baby is in a sleep cycle and not moving much. During quiet periods, the fetal heart rate may not accelerate, which is necessary for a clear result. In these cases, the technician may gently rouse the baby using a small acoustic stimulation device placed on the abdomen, which produces a buzzing sound.
The total time spent in the clinic or hospital will be longer than the monitoring period itself. This appointment time includes check-in, preparing the equipment, placing the monitors, and consulting with the health care provider afterward. Patients should allow for a total appointment time that accommodates these necessary steps.
Step-by-Step: What Happens During the Test
The Non-Stress Test begins with the patient lying down comfortably, usually in a semi-reclined position to ensure adequate blood flow. The abdomen is prepared for the placement of two external monitoring devices, called transducers, which are held in place by elastic belts.
One transducer monitors the fetal heart rate using Doppler technology, while the second device, a tocodynamometer, detects uterine contractions. A conductive gel is applied to the skin to help transmit the signals clearly from the baby’s heart to the monitor.
The nurse or technician adjusts the heart rate monitor until the baby’s heartbeat is clearly audible and the signal is stable. The equipment records the fetal heart rate and uterine activity onto a continuous strip or digital display. The mother is given a handheld button to press every time a fetal movement is felt, helping the team correlate movement with heart rate changes.
Understanding the Test Results
The Non-Stress Test determines if the baby’s heart rate exhibits a healthy response to movement, classified as either reactive or non-reactive. A “reactive” result is reassuring and indicates that the baby is well-oxygenated and neurologically responsive.
To be classified as reactive, the baby’s heart rate must accelerate at least twice within the 20-minute monitoring period. Each acceleration must rise 15 beats per minute above the baseline rate and last for a minimum of 15 seconds, which is the standard for babies after 32 weeks of gestation. A “non-reactive” result occurs if the required heart rate accelerations do not meet these criteria within the 40-minute monitoring period.
A non-reactive result does not automatically signify a problem, as it can be caused by the baby being in a natural sleep cycle or by certain maternal medications. If the test is non-reactive, the health care provider recommends additional tests, such as a biophysical profile, to gather more information about the baby’s overall health.