A Non-Stress Test (NST) is a common prenatal assessment used to monitor the well-being of a developing fetus. It helps healthcare providers understand how a baby is doing by observing the fetal heart rate in response to movement and uterine activity. Understanding the information revealed by an NST, especially regarding uterine contractions, can provide valuable insights for expectant parents.
Understanding the Non-Stress Test
The Non-Stress Test is a non-invasive procedure evaluating fetal health. Its primary purpose is to assess the fetal heart rate in response to the baby’s movements, which indicates adequate oxygen supply. This test is often recommended in the third trimester of pregnancy, particularly for high-risk pregnancies, those that have gone past their due date, or when there are concerns about decreased fetal movement.
During an NST, the pregnant individual typically lies in a reclined position. Two elastic belts with sensors are placed on the abdomen. One sensor, a Doppler ultrasound transducer, monitors the fetal heart rate. The other sensor, known as a tocodynamometer or Toco, detects uterine contractions.
The information from both sensors is displayed on a monitor or printed as a tracing for analysis, usually over 20 to 30 minutes. Patients may also be asked to press a button when they feel the baby move, correlating fetal activity with heart rate changes on the tracing.
The Nature of Uterine Contractions
Uterine contractions involve the tightening and relaxation of the uterine smooth muscle. These muscular actions can occur at various intensities throughout both non-pregnant and pregnant states. During pregnancy, contractions serve multiple functions, from preparing the uterus for labor to facilitating the delivery of the baby.
Two main types of contractions are often discussed in pregnancy: Braxton Hicks contractions and true labor contractions. Braxton Hicks contractions are often described as “practice” contractions; they are typically irregular, unpredictable, and do not increase in intensity or frequency. They usually do not lead to cervical changes and may subside with a change in position or activity. In contrast, true labor contractions are regular, become progressively stronger, longer, and more frequent, and lead to changes in the cervix, such as dilation and effacement. While Braxton Hicks contractions might feel like a general tightening across the belly, true labor contractions can be more painful and may involve sensations in the lower back or abdomen.
Interpreting Contractions on the NST Tracing
On an NST tracing, uterine contractions appear as upward curves or “hills” on the lower line of the graph, while the fetal heart rate is shown on the upper line. The tocodynamometer primarily records the presence, frequency, and duration of these contractions by sensing the pressure changes on the maternal abdomen.
The frequency of contractions is measured from the beginning of one contraction to the beginning of the next. Duration is measured from the start of a single contraction to its end. While the monitor shows when a contraction happens and how long it lasts, external monitors like the tocodynamometer do not accurately measure the true intensity or strength of the contractions. Internal intensity, measured in millimeters of mercury (mmHg), can only be precisely determined with an internal uterine pressure catheter, which is not part of a standard NST. Therefore, the “height” of the hill on the NST tracing reflects the pressure change detected by the external sensor rather than the actual force.
What the NST Results Mean
The overall interpretation of an NST considers both the fetal heart rate patterns and any observed uterine activity. An NST is typically classified as “reactive” or “non-reactive.” A reactive NST is a reassuring sign, characterized by two or more fetal heart rate accelerations (increases of at least 15 beats per minute above the baseline, lasting for at least 15 seconds) within a 20-minute period. This indicates adequate oxygen supply.
A “non-reactive” NST means that the fetal heart rate did not meet the criteria for a reactive test within the monitoring period. This does not necessarily indicate a problem; it could mean the baby was asleep or due to medications. However, a non-reactive result often warrants further evaluation, such as extending monitoring to 40 minutes, or performing additional tests like a biophysical profile or a contraction stress test. Abnormal contraction patterns, such as tachysystole (more than five contractions in 10 minutes) or prolonged contractions (lasting over 90 seconds), are concerning, especially when coupled with non-reassuring fetal heart rate patterns. In such cases, further monitoring or intervention may be considered to ensure fetal well-being.