The Toco number is the numerical output generated by a Tocodynamometer, an external medical device used to monitor uterine contractions during pregnancy and labor. This number, often displayed in arbitrary units or labeled as millimeters of mercury (mmHg), represents the mechanical forces exerted by the contracting uterus. The primary purpose of the Toco number is to provide a standardized, continuous record of uterine activity. This data point, when plotted over time, allows healthcare providers to assess the characteristics of the labor process. Importantly, the Toco number is not a direct measure of pain or contraction strength but rather an indication of abdominal tension changes.
The Tocodynamometer: How Contractions Are Measured
The Tocodynamometer, commonly referred to as the Toco, is an external sensor used in electronic fetal monitoring to track uterine activity. This small, disk-shaped device is secured to the mother’s abdomen, typically over the fundus, the muscular top portion of the uterus where contractions are strongest. The device operates as a pressure transducer, sensing the changes in abdominal contour and tension as the uterine muscle hardens. The sensor converts this mechanical distortion into an electrical signal, which is transmitted to the monitor. The monitor displays the Toco number as a real-time numerical value and plots it on a graph. The resulting tracing appears as a series of bell-shaped curves or “hills,” with the peak of each curve corresponding to the maximum tension detected during that specific contraction. This non-invasive method allows for continuous external monitoring.
Interpreting the Toco Numbers: Frequency and Duration
The Toco numbers and their corresponding waveform tracings are interpreted to determine the two primary parameters of labor: frequency and duration. Frequency is calculated by measuring the time interval between the start of one contraction, marked by the upward slope of the “hill,” and the start of the next contraction. This measurement is expressed in minutes. Duration refers to the length of time a single contraction lasts, measured from the point the Toco number begins to rise above the baseline until it returns to the resting tone. This measurement is expressed in seconds. For example, in the active phase of labor, contractions typically last between 30 and 90 seconds and occur every two to three minutes.
The numerical values displayed on the monitor also offer a general reference for the stage of labor. Braxton Hicks contractions, or practice contractions, usually register a low Toco number, often ranging from 5 to 25 mmHg. In contrast, contractions during active labor often peak between 40 and 70 mmHg, and during the final pushing stage, they may register 50 to 80 mmHg. These numbers, combined with the frequency and duration patterns, allow providers to gauge the effectiveness of uterine activity.
Why Monitoring Toco Numbers is Essential During Labor
Monitoring the pattern of Toco numbers is a fundamental aspect of assessing the overall progression of labor and ensuring the well-being of the fetus. The documentation of contraction frequency and duration helps the medical team determine if labor is advancing at an expected rate. A pattern of contractions that are too infrequent or too short may indicate hypotonic uterine dysfunction, suggesting the need for intervention to augment the labor process. Conversely, monitoring is necessary to identify uterine tachysystole, defined as more than five contractions in a ten-minute window. This rapid pattern reduces the recovery time between contractions, potentially compromising the fetus’s oxygen supply. Tracking these patterns allows healthcare providers to make informed decisions about adjusting medications, such as oxytocin, or timing necessary clinical interventions.
Limitations of External Toco Monitoring
A significant limitation of the external Tocodynamometer is that it only measures the frequency and duration of contractions, not the actual intensity or true intrauterine pressure. The Toco number is an arbitrary unit reflecting the tension of the abdominal wall, not the absolute force generated inside the uterus. Consequently, two patients with the same Toco number may be experiencing contractions of vastly different internal strengths.
The accuracy of the Toco number is also highly sensitive to external factors, making the measurement relative to the individual patient. These factors include:
- Placement of the transducer
- Movement by the mother
- Tightness of the securing belt
- Patient’s body mass index
For a precise measurement of the true pressure or intensity of a contraction, an Intrauterine Pressure Catheter (IUPC) must be used internally, which is a more invasive procedure.