Braxton Hicks contractions are common uterine tightenings often described as “false labor,” which occur as the body practices for childbirth. These sporadic episodes are generally mild and do not result in the progressive changes to the cervix that mark the beginning of true labor. Because these practice contractions can sometimes be confused with the onset of labor, medical monitoring is frequently used to help distinguish between the two types of uterine activity. Understanding how these contractions appear on a monitor strip provides a clear visual distinction for healthcare providers.
How Contraction Monitoring Works
Contraction monitoring is typically performed using an external device called a tocodynamometer, often referred to as a “toco.” This sensor is secured to the mother’s abdomen, usually with a belt, positioned over the area of the uterus where contractions are felt most strongly. The toco works by sensing the pressure changes on the abdominal wall as the uterine wall contracts and hardens.
The device translates these pressure changes into a continuous, wavy line on a paper strip or a screen. This graphical output allows healthcare providers to accurately determine the frequency and the duration of the uterine activity. The toco only provides a relative measure of intensity, as it is an external device measuring pressure through the skin and tissue. The actual, absolute strength of the contraction inside the uterus cannot be precisely determined by this external method.
Visual Signature of Braxton Hicks
On the monitoring strip, Braxton Hicks contractions appear as short, scattered elevations, reflecting their mild and irregular nature. The key visual characteristic is their low amplitude, meaning the peaks of the waves are relatively short, often corresponding to readings in the approximate range of 5 to 25 mmHg. These short peaks indicate that the pressure exerted on the abdominal sensor is minimal.
A distinguishing feature is the lack of a consistent pattern or rhythm in the tracing. The time interval between each bump on the line is inconsistent, and the duration of each individual contraction peak is highly variable. They do not follow the predictable regularity seen in active labor. Furthermore, these practice contractions generally fail to show any progression in their visual appearance; their height and spacing will not systematically increase over a period of observation. The overall tracing often looks disorganized and sporadic, with the pressure line returning to the baseline quickly after each small rise.
Visual Signature of True Labor
True labor contractions create a tracing that is highly organized, rhythmic, and progressively more intense. The visual signature of active labor is marked by a significant increase in the amplitude of the waves, producing peaks noticeably taller than those of Braxton Hicks contractions. During the active phase of labor, these peaks typically correspond to readings in the range of 40 to 60 mmHg, and can climb higher, reaching 50 to 80 mmHg during delivery.
The pattern of true labor is distinctly uniform, showing a consistent, rhythmic interval between the peaks on the tracing. Each contraction appears as a well-defined, bell-shaped curve, with a gradual ascent to the peak and a slow return to the baseline pressure. This consistent shape reflects the coordinated tightening and releasing of the entire uterine muscle.
The most telling visual sign is the progressive change in the tracing over time. As labor advances, the peaks become increasingly taller, reflecting greater intensity, and the waves become wider, indicating longer duration. Simultaneously, the space between the peaks shortens, showing that the contractions are becoming closer together. This unmistakable pattern of increasing strength, length, and frequency is the visual hallmark of effective labor that is actively dilating the cervix.
Factors That Alter Monitor Readings
While the tocodynamometer provides a helpful visual guide, its readings are susceptible to external factors that can distort the tracing and lead to misinterpretation. One significant variable is the patient’s body habitus, particularly a higher maternal body mass index. Increased adipose tissue on the abdomen can dampen the pressure signal, making true contractions appear to have a lower amplitude than their actual internal strength.
Patient movement is another common factor that alters the tracing, as shifting position can cause the external sensor to momentarily lift or misalign. This movement artifact can create artificial, jagged spikes or sudden gaps on the strip, which may be mistaken for a contraction or a loss of signal. Similarly, the tightness and placement of the belt securing the toco are highly influential. If the belt is too loose, even a strong contraction may register as a weak signal. If it is too tight, the baseline pressure may be artificially elevated, making all contractions look stronger than they are. Healthcare providers rely on correlating the monitor’s visual data with the patient’s subjective feeling of the contraction and a manual check of the abdomen.