A Non-Stress Test (NST) is a common prenatal procedure used to evaluate the health of a baby during pregnancy. This non-invasive assessment monitors the baby’s heart rate in response to its movements. It provides important insights into how the fetus is tolerating its environment and helps healthcare providers understand if the baby is receiving enough oxygen and developing as expected.
What a Non-Stress Test Involves
A non-stress test typically occurs in a healthcare provider’s office or a hospital’s prenatal testing area, usually after 28 weeks of pregnancy. The expectant mother lies in a reclined position. Two elastic belts are placed around the abdomen: one monitors the baby’s heart rate, and the other detects uterine contractions, even those not felt.
The baby’s heart rate is recorded on a monitor, often displayed on a screen. During the test, which usually lasts 20 to 40 minutes, the mother might be asked to press a button each time she feels the baby move. This helps correlate fetal movement with heart rate changes. The test is called “non-stress” because it introduces no artificial stress on the baby.
Key Indicators on an NST
Healthcare providers analyze several specific patterns and components recorded during an NST to assess fetal well-being. The baseline fetal heart rate is the average heart rate when the baby is not moving or experiencing contractions. A normal baseline range for a fetus is 110 to 160 beats per minute (bpm); deviations from this range can indicate potential concerns.
Accelerations are temporary increases in the fetal heart rate above the baseline. These are a reassuring sign, indicating a healthy nervous system and adequate oxygenation. For babies at 32 weeks gestation or older, an acceleration is an increase of at least 15 bpm above baseline, lasting at least 15 seconds. For babies under 32 weeks, the criteria are 10 bpm for at least 10 seconds.
Variability refers to the natural fluctuations or oscillations in the fetal heart rate around the baseline. This beat-to-beat variation indicates a healthy fetal nervous system. Moderate variability (6 to 25 bpm) is normal and reflects a well-oxygenated fetus. Reduced variability can suggest reduced oxygen levels or other factors affecting the baby’s central nervous system.
Decelerations are temporary decreases in the fetal heart rate. While some types of decelerations, like early decelerations, can be normal and related to head compression, others may indicate a need for further evaluation. The presence, depth, and duration of decelerations are assessed, as certain patterns can suggest issues like umbilical cord compression or insufficient placental blood flow.
Understanding Your NST Results
NST results are commonly categorized as “reactive” or “non-reactive.” A reactive NST is considered a reassuring sign of fetal well-being. It means that within a 20-minute testing period, the baby has shown at least two accelerations meeting the specified criteria (e.g., 15 bpm increase for 15 seconds for term babies). A reactive test suggests the baby is well-oxygenated and their nervous system is functioning.
Conversely, a non-reactive NST indicates that the baby did not meet the criteria for a reactive test within the initial monitoring period. This does not automatically mean there is a problem. Common reasons for a non-reactive result include the baby being in a sleep cycle. Certain medications taken by the mother can also affect the baby’s activity and heart rate patterns.
If the initial 20-minute period is non-reactive, the test may be extended for an additional 20 minutes, totaling 40 minutes, to account for a fetal sleep cycle. Healthcare providers might also try to stimulate the baby with a small buzzer placed on the abdomen or by having the mother consume a sugary drink. If the test remains non-reactive after these measures, further assessment is needed to determine the baby’s health.
Follow-Up After Your Test
The next steps after an NST depend on the results. If the NST is reactive, it indicates the baby is doing well, and no immediate further action is required beyond routine prenatal care.
If the NST is non-reactive, healthcare providers will recommend additional testing. A common follow-up is a biophysical profile (BPP), which combines an NST with an ultrasound examination. The BPP assesses various aspects of fetal health, including breathing movements, body movements, muscle tone, and amniotic fluid volume. Another follow-up test is a contraction stress test (CST), which evaluates how the baby’s heart rate reacts to uterine contractions.
If NST results are inconclusive, or the baby was not active enough, the healthcare provider may repeat the NST later. These follow-up tests provide more comprehensive information to help determine if the baby is at risk or if the non-reactive result was due to a temporary factor like a sleep state.