The Non-Stress Test (NST) is a simple, non-invasive screening tool widely used in the third trimester of pregnancy to assess fetal well-being. The primary goal of the NST is to evaluate the fetal heart rate and its response to movement. This test confirms the baby is receiving sufficient oxygen supply in the uterus, as a well-oxygenated fetus typically shows a temporary increase in heart rate when it moves.
Why a Non-Stress Test is Performed
The NST is typically performed when closer monitoring of the fetal environment is needed, often starting after 28 weeks of gestation. At this point, the fetal nervous system is mature enough for the heart rate to consistently respond to activity. The test helps determine if the baby is at risk for insufficient oxygenation (fetal hypoxia), which can be linked to placental or umbilical cord issues.
Clinical indications for performing the test frequently involve managing high-risk pregnancies. Maternal conditions like pre-existing or gestational diabetes, high blood pressure (hypertension), or clotting disorders often necessitate regular surveillance. The NST is also routinely ordered for pregnancies that are overdue, generally surpassing 40 or 42 weeks, to ensure the placenta continues to function adequately.
Other reasons include a maternal report of decreased fetal movement, which warrants immediate evaluation. If a previous pregnancy resulted in a stillbirth, or if the current pregnancy involves multiples (twins or more), a non-stress test may be part of a regular monitoring schedule.
The Non-Stress Test Procedure
The non-stress test is a straightforward procedure that generally takes between 20 and 40 minutes. The patient lies comfortably in a semi-reclined position. Two elastic belts, each containing a specialized sensor called a transducer, are secured around the abdomen.
One transducer is positioned to monitor the fetal heart rate, while the second measures the presence or absence of uterine contractions. The fetal heart rate and any detected contractions are recorded on a paper printout or displayed on a monitor, which allows the healthcare provider to analyze the data. The patient may be given a handheld button and asked to press it every time they feel the baby move, correlating movement with the recorded heart rate.
If the fetus is inactive or in a sleep cycle, the test duration may be extended to 40 minutes. To encourage activity, the healthcare provider might use a vibroacoustic stimulator—a small device placed on the abdomen that emits a buzzing sound to gently wake the baby. Consuming a sugary drink or a snack is also sometimes used to stimulate fetal movement.
Interpreting the Test Results
The results of a non-stress test are categorized as either “Reactive” or “Non-Reactive.” A Reactive result is considered reassuring, suggesting the baby is well-oxygenated and healthy. The criteria for a reactive result require the fetal heart rate to temporarily increase, or “accelerate,” in response to movement.
In pregnancies that have reached 32 weeks or later, a reactive result is defined by having two or more heart rate accelerations within a 20-minute period. Each acceleration must rise at least 15 beats per minute above the baseline heart rate and must last for a minimum of 15 seconds. For fetuses under 32 weeks, the criteria are slightly less stringent, requiring an increase of 10 beats per minute for 10 seconds.
A Non-Reactive result means the test did not meet the minimum criteria for acceleration within the initial testing period. This outcome does not automatically mean the baby is in distress, as the fetus may be in a prolonged sleep cycle or affected by maternal medications. However, a non-reactive result suggests a possible reduction in oxygen reserve and necessitates further evaluation.
Next Steps After Non-Reassuring Results
A non-reassuring, or non-reactive, NST result prompts the healthcare team to immediately pursue additional testing to gain a clearer picture of fetal well-being. The most common follow-up is the Biophysical Profile (BPP), which combines the NST with an ultrasound examination. The BPP assesses four additional parameters:
- Fetal breathing movements
- Body movements
- Muscle tone
- The volume of amniotic fluid
Another subsequent test that may be ordered is the Contraction Stress Test (CST), which evaluates how the fetal heart rate responds to mild uterine contractions. These contractions are typically induced either by administering a low dose of the hormone oxytocin or by stimulating the nipples. The CST simulates the reduced oxygen flow that naturally occurs during a contraction to see if the baby can tolerate the brief stress.
Management after a non-reassuring result is based on the combined information from the NST, BPP, and CST, along with the baby’s gestational age and the mother’s clinical status. If the subsequent tests are also concerning, the clinical team may recommend increased surveillance with more frequent testing or continuous fetal monitoring. In situations where immediate and severe risk to the fetus is identified, the course of action may progress to hospitalization, or, if the gestational age permits, an induction of labor or a delivery.