The Non-Stress Test (NST) is a routine, non-invasive procedure used during pregnancy to evaluate the health of the developing fetus. It monitors the fetal heart rate and observes how the rate responds to the baby’s movements. Typically performed during the third trimester, the NST provides medical providers with data on the baby’s oxygen supply and overall condition within the uterine environment.
Purpose and Timing of the Non-Stress Test
A physician orders an NST when there is concern regarding the baby’s environment or the mother’s health status. Common clinical indications include pregnancies complicated by conditions such as gestational diabetes, chronic high blood pressure, or preeclampsia. The test is also frequently used when the expectant mother reports a noticeable decrease in the frequency or strength of fetal movements.
Monitoring often begins when a pregnancy extends beyond the estimated due date (post-term pregnancy) or in cases involving advanced maternal age. The NST is considered accurate and useful starting after 28 weeks of gestation. For high-risk patients, testing often begins around 32 to 34 weeks and may be scheduled weekly or even twice a week.
How the Non-Stress Test is Performed
During the procedure, the mother is positioned comfortably in a semi-reclined chair or bed to prevent pressure on a major blood vessel, which can affect blood flow. Two external monitoring devices are secured to the abdomen using elastic belts. One device is a Doppler transducer that uses sound waves to continuously track and record the fetal heart rate.
The second device, a tocodynamometer, detects and records any uterine contractions or episodes of fetal movement. The test typically lasts about 20 minutes, but it may be extended if the fetus is inactive or in a sleep cycle. The mother is also given an event marker, a button she presses every time she perceives the baby moving, which correlates the movement with the recorded heart rate pattern.
Interpreting the Test Results
The collected data is analyzed to determine if the test is “Reactive” or “Non-reactive,” the two primary results. A Reactive result is considered reassuring, indicating a healthy oxygen supply to the fetus and a functioning central nervous system. To be classified as Reactive, the fetal heart rate must accelerate by at least 15 beats per minute above the baseline rate, and this acceleration must last for 15 seconds.
These accelerations must occur two or more times within the 20-minute monitoring period. A Non-reactive result occurs when the fetal heart rate does not meet these criteria. This may happen if the accelerations were too few, too short in duration, or too small in magnitude.
However, a consistently Non-reactive result raises concern for a potential compromise in the fetal environment, prompting further diagnostic investigation. The baseline fetal heart rate, the average rate when the baby is not moving, is also assessed, usually falling between 110 and 160 beats per minute.
Follow-Up Testing
If the NST result is Non-reactive or inconclusive after the initial 20 minutes, the medical team often attempts to stimulate the fetus. This stimulation can involve the mother drinking juice to elevate her blood sugar or using an acoustic stimulator placed on the abdomen to gently wake the baby. If the test remains Non-reactive after these efforts, further diagnostic steps are typically initiated.
The most common next step is often a Biophysical Profile (BPP), which combines the NST results with an ultrasound assessment of four additional parameters:
- Fetal breathing
- Body movement
- Muscle tone
- The amount of amniotic fluid
Another secondary tool is the Contraction Stress Test (CST), which evaluates the fetal heart rate response to mild, induced uterine contractions. These follow-up tests determine if the baby needs to be delivered or if the pregnancy can safely continue under close monitoring.