The Non-Stress Test (NST) is a common, non-invasive prenatal screening tool performed in the third trimester to assess fetal well-being. It monitors the baby’s heart rate, typically over 20 to 30 minutes, to check for adequate oxygen supply. While the primary objective is evaluating the fetal heart rate, the equipment also detects uterine activity. This means the NST can record contractions the mother may be having, even those she does not feel. Detecting uterine activity provides context for interpreting the baby’s heart rate response to movement and contractions.
The Primary Goal of the Non-Stress Test
The core purpose of the Non-Stress Test is to determine if the baby is receiving enough oxygen. This is assessed by monitoring the fetal heart rate (FHR) in response to movement or contractions. The test is named “non-stress” because it places no external stress on the baby or the mother. It is frequently ordered for high-risk pregnancies, such as those complicated by diabetes or high blood pressure, or when a baby is moving less than usual.
A healthy baby will show an increase in heart rate during periods of activity. The NST looks specifically for “accelerations,” which are temporary increases in the FHR above the normal baseline rate. These accelerations demonstrate that the baby’s nervous system is properly reacting to movement, correlating with sufficient oxygenation. The baseline heart rate for a fetus in the late third trimester typically ranges between 110 and 160 beats per minute.
The test result is categorized as “reactive” or “non-reactive” based on the presence of heart rate accelerations. A reactive result is reassuring, met when there are at least two FHR accelerations, each rising 15 beats per minute above the baseline and lasting for 15 seconds, all within a 20-minute monitoring period. A reactive test suggests adequate blood flow and oxygenation to the fetus.
A non-reactive result means the criteria for a reactive test were not met during the 20-minute period. This result does not automatically indicate a problem, as the baby might simply be asleep or certain medications could be affecting its activity level. However, a non-reactive result requires further evaluation, such as extending the monitoring period or performing additional tests like a biophysical profile, to rule out inadequate oxygen supply.
How Contractions Are Monitored During the NST
The NST equipment involves two external monitoring devices secured to the mother’s abdomen with elastic belts. One device uses ultrasound technology to track the fetal heart rate. The second device, a tocodynamometer or “toco,” is a pressure-sensitive gauge that monitors uterine activity.
The tocodynamometer is typically placed over the uterine fundus, where contractions are felt most strongly. This sensor detects changes in the tension of the abdominal wall caused by the tightening of the underlying uterine muscle. As the uterus contracts, it pushes against the sensor, translating that pressure into a tracing on the monitoring strip.
The toco’s tracing provides a record of the frequency and duration of any uterine contractions. Frequency is measured by the time interval between the start of one contraction and the start of the next. Duration is the length of time the contraction lasts, from beginning to end. This information is used to determine if contractions, even mild ones, are affecting the baby’s heart rate, potentially causing a deceleration.
Limitations of External Contraction Monitoring
While the tocodynamometer successfully records the timing of uterine activity, it has significant limitations regarding the true strength of the contractions. Because it is an external device, the toco measures the pressure exerted on the abdominal wall, not the actual, internal pressure within the uterus. This means the tracing can accurately show when a contraction starts and stops, but the height of the contraction wave on the monitor strip does not directly correspond to its intensity or power.
Several factors can impact the accuracy of the toco reading, leading to an unreliable representation of contraction strength. These include:
- The mother’s body mass.
- The specific placement of the sensor on the abdomen.
- The tightness of the belt.
- A shift in the mother’s position.
- A shift in the baby’s position.
In contrast, an intrauterine pressure catheter (IUPC), an internal monitor inserted into the uterus, is considered the gold standard for measuring the true intensity of contractions. Because the external tocodynamometer cannot quantify the actual force of the uterine muscle, it is not a definitive tool for assessing the progression or effectiveness of labor. The NST provides valuable data on contraction frequency and duration, but it cannot definitively diagnose the strength of labor contractions.