The Non-Stress Test (NST) is a common, non-invasive prenatal screening tool used primarily in the third trimester of pregnancy. It evaluates the health of the developing baby by monitoring the fetal heart rate and its response to movement. This provides important information about the fetal environment and oxygen supply. This assessment helps healthcare providers ensure the baby is thriving.
Why the Non-Stress Test is Performed
The goal of the Non-Stress Test is to check for fetal well-being by observing the baby’s heart rate response to movement. A healthy baby with sufficient oxygen and a functioning nervous system will show a temporary increase in heart rate when active. The NST assesses this physiological response, gauging whether the baby is receiving adequate oxygenation from the placenta.
A doctor typically orders the test when a pregnancy is considered high-risk. Indications include maternal conditions like chronic hypertension, preeclampsia, or diabetes. The test is also performed if the mother reports reduced fetal movement, if the pregnancy is past the due date, or if the baby is measuring small for its gestational age. The NST is generally performed after 28 weeks of gestation, when the baby’s neurological development allows for a reliable heart rate response.
How the Test is Conducted
The Non-Stress Test typically takes 20 to 40 minutes to complete. The mother is placed in a reclining position, often tilted slightly to the left side to optimize blood flow. Two elastic belts with sensors, called transducers, are secured around the abdomen.
One transducer uses ultrasound to track the baby’s heart rate, while the other monitors for uterine contractions. The heart rate tracing is recorded continuously on a screen. The mother presses an event marker button every time she feels the baby move, correlating movement with heart rate changes. If the baby is inactive, an acoustic stimulator may be used on the abdomen to gently rouse the baby.
Understanding Reactive and Non-Reactive Results
The results of the NST are categorized as either reactive or non-reactive. A reactive result is reassuring, indicating the baby is well-oxygenated and the nervous system is functioning appropriately. To be classified as reactive, the fetal heart rate must accelerate a specific number of times during monitoring.
Reactive Criteria
For babies at or beyond 32 weeks of gestation, a reactive result requires two or more heart rate accelerations within a 20-minute window. Each acceleration must increase the heart rate by at least 15 beats per minute above the baseline rate and last for a minimum of 15 seconds. Before 32 weeks, the criteria is 10 beats per minute increase lasting 10 seconds (10×10 criteria).
A non-reactive result occurs when the tracing fails to meet these acceleration criteria within the 20-minute period. This result is non-reassuring and requires further evaluation, but it does not automatically signify distress. Common reasons for a non-reactive result include the baby being in a sleep cycle or the temporary effects of maternal medications.
If the tracing remains non-reactive after the initial 20 minutes, the test may be extended for another 20 minutes, sometimes using vibroacoustic stimulation. Failure to meet the reactive criteria after this extended monitoring suggests the need for immediate additional testing. A non-reactive result may signal that the baby is not receiving adequate oxygen, often due to placental function issues.
What Happens After a Non-Reactive Test
When an NST yields a non-reactive result, the healthcare provider moves to more comprehensive fetal assessment methods. The most common next step is a Biophysical Profile (BPP), which combines the NST with a detailed ultrasound examination. The BPP evaluates four additional parameters of fetal well-being: body movement, muscle tone, breathing movements, and the volume of amniotic fluid.
Another follow-up option is the Contraction Stress Test (CST), which assesses how the baby’s heart rate reacts to uterine contractions. For the CST, mild contractions are induced using medication like oxytocin or through nipple stimulation. These subsequent tests provide a complete picture of the baby’s health to determine if there is an actual compromise. The results of the BPP or CST guide the medical team in deciding whether continued monitoring is appropriate or if intervention, such as inducing labor or performing a cesarean section, is necessary.