A tocodynamometer, or “toco,” is a medical device used during pregnancy and labor to monitor uterine activity. This non-invasive tool provides healthcare providers with information about contractions, which is important for monitoring maternal and fetal well-being. It is a routine part of electronic fetal monitoring, tracking the baby’s heart rate alongside uterine contractions.
What a Toco Number Measures
An external tocodynamometer operates as a pressure-sensitive sensor placed on the mother’s abdomen where contractions are felt. This sensor detects changes in abdominal tension as the uterine muscle tightens and relaxes. It converts these mechanical changes into an electrical signal sent to a monitor.
The device primarily gathers two pieces of information about uterine contractions: their frequency and duration. Frequency refers to how often contractions occur, measured from the start of one to the start of the next. Duration indicates how long each contraction lasts, from its beginning to its end. This information helps understand labor progression and ensures the well-being of the mother and baby.
Understanding the Toco Reading
The information collected by the tocodynamometer is displayed on a monitor as a line graph, usually below the fetal heart rate tracing. This graph shows peaks and valleys, representing the rise and fall of uterine pressure. The lowest point on the graph, between contractions, represents the “baseline” or “resting tone” of the uterus, which is the pressure when the uterus is relaxed, typically around 10-12 mmHg.
As the uterus contracts, the pressure detected by the toco increases, causing the line on the graph to ascend and form a peak. The “toco number” displayed on the monitor, measured in millimeters of mercury (mmHg), corresponds to this detected pressure. This numerical value is a relative measure of pressure changes, not an absolute indicator of the contraction’s actual strength inside the uterus. Healthcare providers observe the overall pattern, shape, and timing of these peaks to assess labor progression, rather than focusing on a single numerical value. For instance, Braxton Hicks contractions might show readings between 5-25 mmHg, while active labor contractions could range from 40-70 mmHg.
Limitations of External Toco Monitoring
The primary limitation of an external tocodynamometer is its inability to accurately measure the true intensity or strength of uterine contractions. While the device registers a pressure change, this “toco number” does not directly reflect the actual force exerted by the uterine muscle. The external monitor detects pressure transmitted through the abdominal wall, which can be influenced by several external factors.
Factors such as the mother’s position, the amount of adipose (fat) tissue on the abdomen, and how tightly the sensor belt is placed can affect the toco reading. For example, a loose belt or significant abdominal tissue might result in lower readings, making contractions appear less intense. Conversely, a tight belt could inflate the numbers. To obtain a precise measurement of contraction strength, an Internal Uterine Pressure Catheter (IUPC) is required, inserted directly into the uterus. However, the external toco remains widely used for its non-invasive nature and effectiveness in monitoring contraction frequency and duration.