The Nonstress Test (NST) is a common, non-invasive screening procedure performed during pregnancy to assess the well-being of the developing baby. This test monitors the baby’s heart rate and how it responds to movement, providing a snapshot of fetal health. A healthy heart rate pattern in response to activity indicates that the baby is receiving adequate oxygenation.
Why a Nonstress Test is Ordered
The primary goal of the NST is to confirm the baby is receiving sufficient oxygen and is thriving within the uterus. Providers typically order this assessment during the third trimester, usually after 28 weeks of gestation. At this stage, the fetal nervous system is mature enough to show the required heart rate responses. For high-risk pregnancies, testing may be performed weekly or even twice weekly until delivery.
Testing is indicated for maternal and fetal conditions that could affect placental function. Common reasons include a pregnancy past the estimated due date or reduced fetal movement. The test is also used when the mother has pre-existing conditions, such as diabetes, high blood pressure, or preeclampsia. Complications like low amniotic fluid levels, multiple babies, or a baby measuring small for gestational age can also prompt the need for an NST.
What to Expect During the Nonstress Test
The procedure is simple and typically takes place in a provider’s office or a hospital’s prenatal testing area, with the mother seated in a reclining chair. Two elastic belts, each holding a small monitoring device called a transducer, are placed around the abdomen. A water-soluble gel is applied to the skin to help the transducers transmit signals effectively.
One transducer tracks the baby’s heart rate, while the other monitors for uterine contractions, even those the mother may not feel. The heart rate tracing is continuously recorded, often displayed on a screen or printed onto paper. The test usually takes around 20 minutes, but it can be extended to 40 minutes if the baby is inactive.
The mother may be asked to press a button every time she feels the baby move. This helps correlate the perceived movement with changes observed in the recorded heart rate. If the baby is in a sleep cycle, the healthcare team may attempt stimulation. This involves having the mother drink a sugary beverage or using a fetal acoustic stimulator, which emits a gentle sound or vibration near the abdomen to rouse the baby.
Interpreting the Test Results
The results of the NST are categorized as either “reactive” or “non-reactive.” A reactive result indicates a reassuring pattern of fetal well-being and is achieved when the baby’s heart rate shows specific increases, called accelerations, in response to movement. After 32 weeks of gestation, reactivity requires at least two accelerations within a 20-minute period.
Each heart rate increase must climb at least 15 beats per minute above the baseline rate and last for 15 seconds. Before 32 weeks, the criteria are less stringent due to the baby’s nervous system immaturity, requiring accelerations of only 10 beats per minute above baseline lasting 10 seconds. The presence of these accelerations suggests the baby is well-oxygenated and the autonomic nervous system is functioning normally.
A non-reactive result means the tracing did not meet the specified criteria for accelerations within the 20-minute window. A non-reactive result does not automatically signify a problem or distress. The most frequent reason is that the baby was simply in a quiet sleep cycle during monitoring. Certain maternal medications may also temporarily suppress the baby’s activity and heart rate response.
When a test is non-reactive, monitoring is often extended for another 20 minutes, totaling 40 minutes, to allow time for the baby to wake up. If the tracing remains non-reactive after this extended time, the care team will recommend further assessment. This typically involves a Biophysical Profile (BPP), which combines the non-reactive NST with an ultrasound to evaluate other measures of fetal health.
The BPP assigns scores based on the baby’s breathing movements, body tone, gross body movement, and amniotic fluid volume. Another follow-up option is the Contraction Stress Test (CST), which assesses how the baby’s heart rate responds to mild uterine contractions. These secondary tests help determine if the non-reactive NST was a transient event or suggests a need for closer surveillance or intervention.