Monitoring during pregnancy and labor involves belts and sensors that provide health professionals with important information about the birthing process. The device that tracks uterine contractions is often abbreviated as “TOCO” on the monitor screen. TOCO is shorthand for Tocodynamometry, a non-invasive technique used to continuously record the timing of uterine muscle activity. This external measurement is a standard part of assessing labor progression and fetal well-being.
Defining the Tocotransducer
The Tocotransducer is the physical, pressure-sensitive sensor used in Tocodynamometry, shaped like a small, circular disk. It is secured to the abdomen using a wide, elastic belt to maintain continuous contact with the skin. The device is positioned over the uterine fundus, the top part of the uterus where contractions are felt most strongly.
The external sensor detects changes in the firmness and shape of the maternal abdomen as the uterine muscle contracts and relaxes. When the uterus tightens, it pushes outward against the abdominal wall, pressing against the sensor’s internal plunger. The Tocotransducer converts this mechanical pressure into an electrical signal that the fetal monitor translates into a visual tracing and a numerical reading.
What the TOCO Reading Actually Represents
The number displayed on the monitor, or the height of the peaks on the printed strip, indicates the relative change in external abdominal pressure, not the actual strength of the contraction inside the uterus. The reading is highly dependent on factors like sensor placement, belt tightness, and the amount of tissue between the uterus and the sensor. Therefore, a reading of “80” may represent a strong contraction for one patient but only a moderate one for another.
The most reliable information provided by the TOCO is the frequency (how often contractions occur) and duration (how long each contraction lasts). The TOCO reading cannot accurately measure the true intensity or intrauterine pressure in millimeters of mercury (mmHg), unlike an invasive internal catheter. Because of this limitation, health professionals must also palpate the abdomen and talk with the patient to accurately gauge the actual strength of the contractions.
Understanding the Contraction Waveform
The data from the Tocotransducer is displayed as a wavy line or tracing, often resembling a series of hills on the monitor strip. When the uterus is at rest between contractions, the line remains at the baseline or resting tone, the lowest point of the tracing. As a contraction begins, the line rises gradually, showing the build-up of pressure.
The highest point of the “hill” is known as the acme or peak, representing the maximum external pressure exerted during the contraction. Following the peak, the line descends back down to the baseline as the uterine muscle relaxes. This complete wave shape allows for the precise measurement of contraction timing.
To determine the duration, a professional measures the time from the point where the line first leaves the baseline until it returns. Frequency is calculated by measuring the time interval from the beginning of one contraction’s peak to the beginning of the next. Analyzing these measurements helps determine if the contractions are effectively progressing labor.
Why TOCO Monitoring is Essential
TOCO monitoring is an integral part of Cardiotocography, the simultaneous recording of both uterine activity and the fetal heart rate (FHR). One common application is during a Non-Stress Test (NST), where the baby’s heart rate is monitored to see how it responds to movement and mild uterine activity. During active labor, the TOCO tracing provides the necessary time stamp for every contraction, which is a stressful event for the fetus.
The timing information from the TOCO is used to interpret the FHR tracing, allowing the care team to see if the baby’s heart rate drops or accelerates in response to uterine tightening. For instance, an FHR drop immediately after a contraction suggests a different cause than a drop occurring during the contraction itself. The TOCO provides the context needed to evaluate how well the fetus is tolerating the labor process and whether intervention is required.