The Non-Stress Test (NST) is a common, non-invasive prenatal test performed during pregnancy to assess the well-being of the developing fetus by monitoring its heart rate patterns.
Understanding the Non-Stress Test
The Non-Stress Test measures the unborn baby’s heart rate in response to its movements. It’s called “non-stress” because it observes the baby’s natural activity without inducing stress.
A primary goal of the NST is to determine if the baby is receiving sufficient oxygen. When a healthy baby moves, its heart rate should temporarily increase, similar to how an adult’s heart rate rises with activity. This response indicates adequate oxygenation and a healthy nervous system.
Healthcare providers may recommend an NST for various reasons, particularly in the third trimester, after 28 weeks of gestation. Common scenarios include concerns about decreased fetal movement, a pregnancy extending past the due date, or if the pregnant individual has medical conditions such as diabetes or high blood pressure. It is also used in high-risk pregnancies or if there were complications in a previous pregnancy.
The Procedure
A Non-Stress Test is typically performed in a healthcare provider’s office or a hospital’s prenatal testing area. The pregnant individual usually lies in a comfortable, semi-reclined position on an exam table.
During the test, two elastic belts are placed around the abdomen. One monitors the baby’s heart rate, and the other detects uterine contractions.
The test usually lasts between 20 and 40 minutes. The pregnant individual may be asked to press a button each time they feel the baby move, correlating heart rate changes with activity. If the baby is not active, a healthcare professional might use a gentle buzzer or encourage a snack to stimulate movement.
Interpreting the Results
The results of a Non-Stress Test are categorized as either “reactive” or “non-reactive.” A “reactive” NST indicates a healthy, well-oxygenated fetus. This occurs when the baby’s heart rate shows two or more accelerations within a 20-minute period. Each acceleration means the heart rate increases by at least 15 beats per minute above the baseline and lasts for at least 15 seconds, particularly after 32 weeks of gestation.
Conversely, a “non-reactive” result means the criteria for a reactive test were not met. This does not automatically signify a problem. A non-reactive result might occur if the baby is in a sleep cycle, is less active, or if maternal medications affect its activity.
When a non-reactive result occurs, further evaluation is typically recommended. This might involve extending the NST for a longer duration, sometimes up to 120 minutes, to allow for more observation. Other follow-up tests could include a biophysical profile (BPP), which uses ultrasound to assess additional indicators of fetal well-being, such as breathing, movement, muscle tone, and amniotic fluid volume. In some instances, a contraction stress test (CST) might be performed, which examines how the baby’s heart rate responds to mild uterine contractions.
Safety and Common Questions
The Non-Stress Test is a safe, non-invasive procedure with no known risks to the pregnant individual or the baby. It uses sound waves to monitor the baby’s heart rate, rather than radiation.
No special preparation is generally needed before an NST, though a light snack or sugary drink may be suggested to encourage fetal activity. The test is not painful, but lying still with monitors strapped to the abdomen may cause slight discomfort.
The frequency of NSTs varies depending on the specific medical situation and the healthcare provider’s recommendation. For some high-risk pregnancies, these tests may be performed weekly or even twice a week to ensure continuous monitoring of the baby’s condition.