TOCO on a monitor stands for tocodynamometer (sometimes called a tocotransducer), which is the sensor that tracks your uterine contractions during pregnancy and labor. It’s one of two main components on a fetal monitor. The other sensor tracks your baby’s heart rate using ultrasound. Together, these two readings give your care team a real-time picture of how your labor is progressing.
How the TOCO Sensor Works
The TOCO transducer is a small, disc-shaped device strapped to your abdomen with an elastic belt. It works on a surprisingly simple principle: when your uterus contracts, your abdominal wall pushes forward slightly. That forward displacement presses against a strain gauge in the center of the sensor, which converts the pressure into a signal displayed on the monitor as a wave-like pattern.
The sensor is placed over your upper uterus (the fundus), typically where it feels firmest. In most cases, that’s over the baby’s buttocks when the baby is head-down. Your nurse may press on your abdomen in a few spots before strapping it on to find the best position for picking up contractions clearly.
What the Numbers Mean
The TOCO reading is displayed in relative units, not in a standardized pressure measurement. The numbers you see on the screen (often ranging from 0 to 100) don’t correspond to a specific amount of force. They show a pattern: the line rises when a contraction happens and falls when it ends. What matters clinically is the shape of that wave, how often the peaks come, and how long each one lasts.
Before monitoring begins, your nurse sets a baseline by adjusting the belt tension so the resting reading sits at roughly 20 to 25 units between contractions. This “zeroing” step gives the monitor a reference point. If you shift positions or the belt loosens, the baseline can drift, which is why nurses periodically readjust it.
What TOCO Can and Cannot Tell You
The TOCO reliably shows two things: how frequently your contractions are coming and how long each one lasts. These are the most important pieces of information for tracking labor progress. Normal contraction frequency is five or fewer contractions in a 10-minute window, averaged over 30 minutes. More than five in that window is called tachysystole, which your team will watch closely because it can affect blood flow to the baby.
What the TOCO cannot do is measure how strong your contractions actually are. The height of the peaks on the screen depends on how tightly the belt is fastened, where the sensor sits, and your body type. A contraction that reaches 80 on one person’s monitor isn’t necessarily stronger than one reaching 60 on someone else’s. If your care team needs precise intensity measurements, they use an internal uterine pressure catheter (IUPC), a thin tube placed inside the uterus that reads pressure directly in millimeters of mercury.
Factors That Affect Accuracy
Several things can make the TOCO less reliable. The most significant is body mass index. Research published in Reproductive Sciences found that patients classified as obese were 82% less likely to have contractions detected by the TOCO compared to patients with a lower BMI. The reason is straightforward: the sensor works by detecting changes in abdominal contour, and a thicker abdominal wall dampens that signal. In these cases, contractions may be happening but simply not registering on the monitor.
Gestational age also plays a role. The TOCO becomes less effective before about 25 weeks of pregnancy, when the uterus is smaller and contractions produce less abdominal displacement. Movement, breathing, coughing, and even laughing can create spikes on the tracing that look like contractions but aren’t. If the belt slips out of position or loosens, the signal can fade or disappear entirely.
External vs. Internal Monitoring
The TOCO is an external monitor, meaning nothing goes inside your body. It’s noninvasive, painless, and the default choice for most laboring patients. You’ll feel the belt around your abdomen, and you may need to stay relatively still for the clearest readings, though wireless versions allow more movement.
An internal monitor (IUPC) is used less frequently and only when your membranes have already ruptured. It provides exact contraction pressure readings, which can be useful when labor isn’t progressing as expected or when your team is adjusting medication to strengthen contractions. The trade-off is that it’s more invasive and carries a small risk of infection. For most straightforward labors, the TOCO provides all the contraction information your team needs.
Reading the Monitor Screen
A standard fetal monitor displays two tracings stacked on top of each other. The top tracing shows the baby’s heart rate, measured in beats per minute by the ultrasound transducer. The bottom tracing is the TOCO output, showing your contraction pattern. Each peak on the bottom line represents one contraction. The time between peaks tells you how far apart your contractions are, and the width of each peak shows how long each contraction lasted.
A contraction lasting less than 45 seconds or more than 3 minutes is generally considered outside the typical range. Your nurse watches both tracings together, looking at how the baby’s heart rate responds during and after each contraction. That relationship between the two lines is one of the most important things the monitor reveals.