The Non-Stress Test (NST) is a common prenatal screening tool used in the later stages of pregnancy to check on the fetus’ well-being. This non-invasive procedure monitors the fetal heart rate and its response to movement, which indicates the baby’s oxygen supply. The name “non-stress” highlights that the test places no physical strain on the mother or the fetus. Understanding the difference between a “reactive” and “non-reactive” result helps guide next steps for monitoring the pregnancy.
The Purpose of the Non-Stress Test
A physician orders an NST when concerns arise regarding the environment and support for the fetus within the womb. The test is designed to determine if the baby is receiving sufficient oxygen, which is reflected in a healthy heart rate pattern. This screening is generally performed after 28 weeks of gestation, as the fetal nervous system is mature enough to show the expected heart rate response.
The test is frequently recommended for higher-risk pregnancies due to underlying maternal conditions. These include gestational diabetes, chronic high blood pressure, or clotting disorders that might affect placental function. It is also used when a mother reports a reduction in the baby’s usual movement patterns or when a pregnancy extends past the due date.
Understanding the Procedure and Equipment
The non-stress test is a straightforward procedure typically lasting between 20 and 40 minutes, performed while the mother reclines comfortably. Two separate monitoring devices, secured to the abdomen, are used to collect the necessary data. One device uses Doppler technology to track the fetal heart rate and spontaneous movements, while the second, called a tocodynamometer, detects and records any uterine contractions.
The heart rate and contractions are printed onto a tracing. The patient may be given a button to press when they feel the fetus move, helping the provider correlate movement with heart rate changes. If the fetus is inactive, a nurse may use a gentle acoustic stimulator to wake them up.
Deciphering the Results: Reactive vs. Non-Reactive
Interpreting the NST involves assessing the baseline fetal heart rate, which normally ranges between 110 and 160 beats per minute (bpm), and observing for specific increases known as accelerations. The test is considered a healthy result, or Reactive, when the heart rate temporarily speeds up from that baseline in response to movement.
For pregnancies at or past 32 weeks, a Reactive result requires at least two accelerations within a 20-minute period. Each acceleration must peak at least 15 bpm above the baseline and last for a minimum of 15 seconds. For pregnancies under 32 weeks, the criteria are slightly less stringent, requiring an increase of 10 bpm for 10 seconds.
A Non-Reactive result is given when the tracing fails to meet these acceleration criteria. This result does not automatically signify a problem, as a non-reactive test is common if the fetus is in a sleep cycle or if the mother has taken certain medications. The test may be extended for an additional 20 minutes to allow the baby to wake up or respond to stimulation. The provider also looks for decelerations (drops in heart rate), as these can be a more concerning finding. Although non-reactive results have a high false-positive rate, the result necessitates further investigation to rule out potential issues with oxygen supply.
Next Steps Following a Non-Reactive Test
When an NST is non-reactive, the healthcare team typically initiates follow-up testing to gather more comprehensive data on fetal well-being. The most common next step is a Biophysical Profile (BPP), which combines the NST with an ultrasound examination. The BPP assesses several indicators of fetal health:
- Breathing movements.
- Body movement.
- Muscle tone.
- The volume of amniotic fluid.
Each of these four ultrasound components, along with the NST result, is scored and totaled to determine a final health score. Alternatively, a Contraction Stress Test (CST) may be performed. This test observes how the fetal heart rate responds to mild uterine contractions, which can be induced by nipple stimulation or medication.