A non-stress test (NST) is a common procedure used in late-stage pregnancy to check the fetus’s overall health. During monitoring, the equipment frequently detects uterine muscle activity that the pregnant individual may not perceive. This often causes confusion when a healthcare provider mentions “contractions” appearing on the readout. This article clarifies how mild uterine tightenings register on the monitoring equipment and what that activity means during an NST.
What is a Non-Stress Test
The non-stress test is a straightforward, non-invasive method that assesses fetal health by monitoring the heart rate. It is termed “non-stress” because it does not involve inducing contractions or placing external strain on the mother or baby. The goal of the test is to observe the fetal heart rate’s behavior during periods of movement. A healthy, well-oxygenated fetus shows a temporary increase in heart rate when it moves.
The test utilizes two external monitoring devices secured to the abdomen with elastic belts. One device is an ultrasound transducer that tracks and records the fetal heart rate (FHR). The second device is a tocodynamometer, often called a toco, which is a pressure sensor. The toco monitors and records uterine muscle activity, even if the activity is too mild to be felt. Both readings are displayed simultaneously on a continuous printout called a cardiotocograph.
Understanding Braxton Hicks Contractions
Braxton Hicks contractions are often described as “practice contractions” or false labor because they prepare the uterus for true labor without causing cervical change. These uterine tightenings are a normal physiological event, commonly noticed in the third trimester. They are characterized by an irregular and non-rhythmic pattern.
A key difference from true labor is that these contractions are relieved by a change in activity, such as walking, resting, or hydration. They are usually mild, uncomfortable abdominal tightening rather than increasingly painful sensations that wrap from the back to the front. Unlike the progressively stronger and more frequent contractions of labor, Braxton Hicks contractions remain sporadic and inconsistent in duration and intensity.
How Braxton Hicks Appear on the Monitor
Braxton Hicks contractions are consistently detected and recorded by the tocodynamometer during an NST. The toco is designed to measure the external pressure changes caused by the tightening of the uterine muscle. Even if a contraction is too mild to be consciously felt, the sensitive sensor registers the pressure change on the abdominal wall.
On the printout, these contractions appear as gentle, broad, wave-like peaks on the lower tracing line, which represents uterine activity. Their intensity is markedly lower than that of active labor contractions, registering in the range of 5 to 25 millimeters of mercury (mmHg). In contrast, true labor contractions often reach 40 to 60 mmHg or higher. The irregular spacing and low pressure reading visually confirm the presence of these practice tightenings.
Interpreting Uterine Activity During the NST
While the tocodynamometer accurately records Braxton Hicks contractions, the primary focus of the non-stress test remains the fetal heart rate tracing. The clinical goal is to determine if the heart rate accelerates appropriately in response to movement, which indicates adequate fetal oxygenation. Therefore, the presence of mild, irregular uterine activity is considered a normal finding.
The detected uterine activity becomes clinically significant only if it influences the fetal heart rate pattern. For example, a clinician notes if a contraction, regardless of its origin, is followed by a non-reassuring decrease in the fetal heart rate. Furthermore, if the toco records frequent, regular contractions—such as four to six or more in an hour—it may raise a concern for potential preterm labor, prompting further evaluation beyond the standard NST scope.