A Non-Stress Test (NST) is a common, non-invasive prenatal screening tool used in the third trimester to monitor fetal well-being. The primary focus is evaluating the baby’s heart rate patterns. However, the equipment simultaneously records any activity from the mother’s uterus, leading to questions about whether it can detect contractions. This dual monitoring system provides valuable information on both fetal health and uterine status.
What is the Non-Stress Test (NST)?
The Non-Stress Test’s main purpose is to determine if the fetus is receiving adequate oxygen and blood flow. This assessment observes how the baby’s heart rate changes in response to its own movements. In a healthy fetus, movement causes the heart rate to temporarily increase, a process known as acceleration.
The test typically lasts for 20 to 30 minutes, during which the baby’s heart rate is continuously recorded. A result is considered “reactive” if the fetal heart rate accelerates two or more times within the 20-minute period. For a full-term baby, this acceleration must increase the heart rate by at least 15 beats per minute above the baseline and last for at least 15 seconds. If these criteria are not met, the result is classified as “non-reactive,” which may warrant further testing.
The Role of Uterine Monitoring During an NST
Contraction detection is a routine part of the Non-Stress Test procedure. To perform the NST, two separate external monitors, called transducers, are secured to the mother’s abdomen with elastic belts. One transducer monitors the fetal heart rate, while the other tracks uterine activity.
This second device is a pressure sensor called a tocodynamometer, or “toco.” The toco is placed on the fundus to detect changes in abdominal tension, as this is where contractions are strongest. When the uterine muscle tightens, the pressure against the toco increases, and this activity is recorded on the monitoring strip. The toco effectively measures the frequency and duration of uterine tightening. However, it cannot accurately measure the true intensity of the contraction, as the reading is influenced by the mother’s body type and belt tightness.
Interpreting Contraction Activity During the Test
The uterine activity recorded during an NST often reveals contractions the mother does not feel, typically Braxton Hicks contractions. These mild, irregular contractions are common in the second and third trimesters and do not indicate that labor is imminent. The monitoring strip provides a visual pattern of this activity, which is assessed alongside the fetal heart rate data.
The significance of contractions during an NST lies in their potential effect on the baby’s heart rate. Clinicians observe whether the fetal heart rate pattern changes in relation to the timing of detected uterine activity. If frequent or strong contractions occur alongside concerning fetal heart rate patterns, this may signal a need for closer observation or a change in management. Detecting sustained, regular contractions is an important data point that may prompt the provider to order a more detailed assessment, such as a Contraction Stress Test or a Biophysical Profile.