The Nonstress Test (NST) is a common, non-invasive prenatal screening tool used to evaluate the well-being of the developing fetus. It is named for its safe nature, as it places no physical stress on the mother or the baby. By monitoring the baby’s heart rate over a short period, healthcare providers gain insight into the fetal environment and its response to movement. The timing of this test depends on specific maternal and fetal health factors, as it is not a universal requirement for every pregnancy.
What the Nonstress Test Monitors
The purpose of the Nonstress Test is to assess the fetal heart rate (FHR) and its relationship with fetal activity. A specialized device tracks the FHR, looking for temporary increases, known as accelerations, that correspond to movement. These accelerations are a direct indicator of a healthy, appropriately oxygenated central nervous system. This physiological response suggests that blood flow and oxygen supply from the placenta are adequate. The absence of expected accelerations may suggest the fetus is in a quiet sleep cycle or that there is a concern regarding oxygen levels.
When Testing Becomes Necessary
The timing for receiving a Nonstress Test is highly individualized and depends on the presence of risk factors rather than a routine schedule. Providers usually order the NST after the baby’s nervous system is mature enough to consistently demonstrate heart rate reactivity, typically after 28 weeks of gestation. For high-risk pregnancies, testing often begins weekly or twice weekly around 32 to 34 weeks.
Testing is warranted when a mother has a chronic medical condition that could affect placental function, such as pre-existing diabetes or chronic hypertension. The NST is also used when concerns arise about the baby’s growth, such as with intrauterine growth restriction. A sudden decrease in fetal movement reported by the mother is a direct trigger for an immediate Nonstress Test to assess the baby’s current condition.
Other indications include carrying multiple fetuses, having a pregnancy that extends past the due date (post-dates pregnancy), or the presence of abnormal amniotic fluid levels. The test serves as a surveillance tool to determine if the baby can safely remain in the uterus. The decision to initiate testing is based on weighing the specific risks of the pregnancy against the gestational age of the baby.
What Happens During the Nonstress Test
The procedure is simple and typically performed in an outpatient setting, lasting between 20 and 40 minutes. You will be positioned comfortably, usually tilted slightly to the left to optimize blood flow. A technician secures two elastic belts around your abdomen, each holding a transducer; one monitors the fetal heart rate and movement, while the second monitors for uterine contractions. You may be given a handheld button to press whenever you feel the baby move, which correlates the movement with the recorded heart rate tracing. If the baby is quiet or asleep, the technician may use an acoustic stimulator to encourage activity.
Interpreting Your NST Results
The results of the Nonstress Test are classified into two categories: reactive or non-reactive. A reactive result is reassuring, indicating fetal well-being and confirming the baby is adequately oxygenated. This result is defined by observing at least two FHR accelerations—each rising 15 beats per minute above the baseline and lasting for 15 seconds—within a 20-minute monitoring period (after 32 weeks). A non-reactive result means the criteria for reactivity were not met, but this does not automatically signal distress, as the baby may be in a sleep cycle or the mother may have taken certain medications. When a test is non-reactive, further assessment is required, often involving extending the monitoring period to 40 minutes or proceeding to a detailed evaluation, such as a Biophysical Profile.