A nonstress test (NST) is a common prenatal screening performed during pregnancy. This safe and painless procedure measures the baby’s heart rate and its response to movement. Its purpose is to assess the baby’s overall well-being and ensure they are receiving adequate oxygen without causing stress to the mother or fetus.
What is a Nonstress Test?
The “nonstress” aspect highlights its non-invasive nature, as it places no additional physical stress on the pregnant person or the baby. During an NST, the primary focus is on measuring the baby’s heart rate in response to their own movements. A healthy baby’s heart rate naturally increases with movement, similar to an adult’s, suggesting the baby is well-oxygenated.
The test also monitors for uterine contractions. If the baby’s heart rate does not accelerate as expected, it may suggest that the baby is not getting sufficient oxygen, or they might simply be asleep.
When an NST is Typically Performed
Healthcare providers recommend a nonstress test during the third trimester of pregnancy, after 28 weeks of gestation. The baby’s heart rate is developed enough to respond to movements around this time. While some tests occur from 28 weeks, it becomes more common from 32 weeks onwards.
An NST may be recommended for various reasons. These include maternal health conditions, such as diabetes, high blood pressure (including preeclampsia), or heart and clotting disorders. If a pregnancy extends past the estimated due date, particularly beyond 42 weeks, an NST may be performed to monitor the baby’s health.
A reported decrease in fetal movement by the mother is another frequent reason for an NST, as it can be a sign that the baby needs closer monitoring. In cases of previous pregnancy complications or a history of stillbirth, an NST helps provide reassurance and ongoing assessment. For multiple pregnancies, such as twins or triplets, an NST might be regularly scheduled due to the increased monitoring needs.
Concerns regarding the baby’s growth, if they are measuring small for gestational age, or issues with amniotic fluid levels (either too much or too little) can also lead to an NST recommendation. Additionally, if there is an Rh incompatibility between the mother’s and baby’s blood types, an NST may be performed.
What Happens During Your NST
During a nonstress test, you will lie in a comfortable, reclining position on an exam table. A healthcare provider will apply a special gel to your abdomen. Two elastic belts are then placed around your belly. One belt holds a transducer that monitors the baby’s heart rate, while the other detects any uterine contractions you might be having, even those you may not feel.
The baby’s heart rate and any contractions are recorded on a monitor, often displayed on a screen or printed out. You may be asked to press a button each time you feel the baby move, which helps correlate the movements with heart rate changes. The test lasts between 20 to 40 minutes, but it can take longer if the baby is asleep or not very active. If the baby is not moving, the technician may use a gentle buzzer-like device on your abdomen to wake the baby, or you might be asked to drink something sugary or have a snack to encourage activity.
Upon completion, the belts are removed, and the gel is wiped off. The results are available immediately and are classified as either “reactive” or “non-reactive.” A “reactive” result indicates that the baby’s heart rate increased appropriately with movement, with two or more accelerations within a 20-minute period, signifying a healthy response. A “non-reactive” result means the heart rate did not increase as expected, which could be due to the baby being asleep, immature, or other factors. A non-reactive result does not necessarily mean there is a problem, but it may lead to further assessments, such as a biophysical profile or a contraction stress test, to gather more information.