Braxton Hicks contractions, often described as “practice contractions,” are common during the second and third trimesters of pregnancy. These uterine tightenings do not result in cervical change, but they can be difficult to differentiate from the onset of true labor. Monitoring these contractions is necessary, especially when preterm labor is a concern. Medical staff differentiate between these types of uterine activity by observing the output of an external monitoring device. This tracing provides an objective visual record, allowing providers to analyze patterns of uterine activity to determine if a patient is experiencing false labor or true labor.
How External Contraction Monitors Work
The external device used to measure uterine activity is called a tocodynamometer, or “toco.” This pressure-sensitive transducer is secured onto the abdomen, typically over the uterine fundus, using an elastic belt. The toco measures changes in the tension of the abdominal wall caused by the tightening of the underlying uterine muscle.
When the uterus contracts, pressure on the abdominal wall increases. The toco translates this physical force into an electrical signal, which is displayed on a screen or printed as a continuous line, known as a tracing. This graph provides a visual representation of uterine activity over time.
The vertical Y-axis of the tracing measures the relative intensity of the contraction, often scaled in millimeters of mercury (mmHg). The external toco provides an indirect measurement of uterine pressure, recording only relative pressure changes. While it accurately indicates the duration and frequency of contractions, the absolute strength (mmHg) displayed is only an approximation of the actual pressure inside the uterus.
Interpreting Braxton Hicks Tracings
When Braxton Hicks contractions appear on the monitor, their visual characteristics differ distinctly from true labor. The contractions register as low-amplitude peaks on the tracing line, typically measuring approximately 5 to 25 mmHg on the monitor’s scale. Visually, these peaks resemble gentle rolling hills rather than the sharp, elevated mountains seen in active labor.
The entire waveform, from the start of the rise to the return to the baseline, is generally short in duration, reflecting the mild nature of the tightening sensation. A primary feature of the Braxton Hicks tracing is its profound irregularity in timing. The peaks are sporadic and unpredictable, showing long, inconsistent gaps without a repeating pattern.
Between these intermittent contractions, the tracing line consistently returns to the flat baseline. This indicates that the uterine muscle is fully relaxing after each tightening episode. The flat baseline between peaks is a strong indicator that the contraction activity is not leading to progressive cervical change.
Key Differences: True Labor Patterns
In contrast to the gentle and erratic nature of Braxton Hicks activity, true labor contractions display a highly coordinated and progressive pattern. The amplitude of the peaks is significantly higher than those seen with practice contractions. During the active phase of labor, these peaks typically reach an intensity between 40 and 60 mmHg, and can climb as high as 80 to 100 mmHg in later stages.
True labor peaks are sustained for a longer period, reflecting a greater duration of uterine muscle tightening. While Braxton Hicks contractions are brief, true labor contractions generally last between 30 and 70 seconds or more. The tracing line remains elevated for this extended time before returning toward the baseline.
The rhythm of true labor contractions is the most definitive characteristic, establishing a regular and predictable frequency. The peaks begin to appear at steadily decreasing intervals, establishing a clear pattern, such as occurring every five minutes. This regularity confirms that the uterine activity is coordinated and progressing.
As true labor advances, a subtle change occurs in the baseline tone of the uterus. Unlike the flat baseline between Braxton Hicks contractions, the resting tone may begin to rise slightly. This elevated baseline indicates that the uterus is not fully relaxing between contractions, signaling a higher level of sustained muscular tension necessary for progressive cervical dilation.