A Non-Stress Test (NST) is a common, non-invasive screening tool used during the late stages of pregnancy to assess fetal well-being. This test monitors the baby’s heart rate to see if it reacts normally to movement, confirming the fetus is adequately oxygenated and healthy. It is frequently ordered when specific risk factors are present, including advanced maternal age, defined as being 35 years or older at the time of delivery. The NST provides an early warning signal or peace of mind, making it a routine part of increased surveillance for expectant mothers.
The Purpose and Procedure of the Non-Stress Test
The term “non-stress” means no physical stress is placed on the fetus or the mother during the test. The primary goal of the NST is to observe the coupling between the baby’s central nervous system and its cardiovascular system, which is a key indicator of oxygen supply. A healthy fetus should demonstrate a temporary increase in heart rate in response to its own movements, much like an adult’s heart rate increases with activity.
The procedure involves placing two external monitors, secured with elastic belts, onto the mother’s abdomen while she reclines comfortably. One monitor tracks the fetal heart rate (FHR), and the other records any uterine contractions. The test typically lasts between 20 and 40 minutes to account for the baby’s natural sleep-wake cycles. The mother may also be given a button to press each time she feels the baby move, helping the technician correlate movement with heart rate changes on the tracing.
Why Advanced Maternal Age Requires Monitoring
Pregnancies in women aged 35 and older are classified as advanced maternal age and are associated with an elevated risk profile, particularly as the pregnancy progresses into the third trimester. This classification acknowledges statistical risks that necessitate proactive management. Healthcare providers recommend regular surveillance with tools like the NST to monitor for potential complications.
These complications include a higher potential for placental insufficiency, which reduces the oxygen and nutrient supply to the fetus. Advanced maternal age is also linked to an increased risk of gestational hypertension and preeclampsia, which can impact blood flow to the placenta. Routine monitoring is a preventative measure, as the risk of stillbirth also increases after age 35.
Interpreting Non-Stress Test Results
The results of a Non-Stress Test are categorized into two primary outcomes: Reactive or Non-Reactive. A Reactive result is reassuring, indicating the baby is well-oxygenated and the fetal central nervous system is functioning. To be deemed reactive, the tracing must show a specific pattern of fetal heart rate accelerations within the monitoring period.
For a term pregnancy, a reactive test requires two or more FHR accelerations. These must peak at least 15 beats per minute (bpm) above the baseline heart rate and last for a minimum of 15 seconds. These accelerations must occur within a 20-minute window, demonstrating the expected heart rate response to fetal movement. If the test is non-reactive, it means these specific criteria were not met.
A non-reactive result does not automatically signify fetal distress, as the baby may simply have been in a deep sleep cycle during the 20-minute test. The test is often extended for another 20 minutes, or a gentle sound stimulus may be used to try and wake the baby. If the criteria are still not met, the result is classified as persistently non-reactive and requires further investigation.
Next Steps Following an Ambiguous Result
When an NST is persistently non-reactive or shows concerning patterns, healthcare providers gather more information about the baby’s health. Follow-up determines whether the result is due to a temporary state, like a sleep cycle, or a more serious issue. The most common secondary assessment is the Biophysical Profile (BPP).
The BPP combines the NST with a detailed ultrasound examination to assess several physiological parameters. This assessment looks at five components, each assigned a score:
- Fetal breathing movements
- Gross body movements
- Muscle tone
- Amount of amniotic fluid
- The Non-Stress Test result
A Contraction Stress Test (CST) may also be performed. The CST monitors the fetal heart rate’s response to mild uterine contractions, which are induced by administering oxytocin or through nipple stimulation. This test assesses how the baby tolerates the brief reduction in blood flow during a contraction, offering a diagnostic tool for placental function.