A toco is a small pressure sensor placed on your belly during pregnancy or labor to track your contractions. Short for “tocodynamometer,” it’s one of the two sensors that make up a standard fetal monitor. The other sensor tracks your baby’s heart rate. Together, they produce the paper strip (or digital readout) that your care team watches throughout labor.
How a Toco Works
The toco is a round, disc-shaped device held against the top of your uterus (the fundus) by an elastic belt. When your uterus contracts, it rises and pushes outward against your abdominal wall. That pressure change pushes against the sensor, which converts the movement into a signal displayed on the monitor. Between contractions, the pressure drops back down, creating the wave-like pattern you see on the screen or printed strip.
Because it sits on the outside of your body, the toco is completely noninvasive. There are no needles, no insertion, and no discomfort beyond the snugness of the belt. A nurse or midwife will reposition it if you shift in bed or if the signal gets weak, which is why you might notice them adjusting the belt throughout labor.
What It Can and Cannot Measure
A toco reliably picks up two things: how often your contractions are coming (frequency) and roughly how long each one lasts (duration). These are the details your care team uses to assess whether labor is progressing normally. For example, contractions coming every two to three minutes and lasting about 60 seconds suggest active labor is well underway.
What a toco cannot tell you is how strong your contractions actually are. The numbers on the screen might read 60 during one contraction and 80 during the next, but those values reflect how much pressure the sensor detects at the skin surface, not the actual force inside your uterus. Factors like how tight the belt is, where exactly the sensor sits, and your body composition all change the reading. So while the peaks and valleys on the tracing show the rhythm of your contractions, the height of those peaks doesn’t reliably indicate intensity.
If your care team needs precise pressure measurements, typically when labor isn’t progressing as expected, they may switch to an internal monitor. This is a thin, flexible catheter placed inside the uterus alongside the baby, and it directly measures the pressure of each contraction in a way the external toco cannot. Most labors don’t require this step.
Reading the Monitor Strip
The standard fetal monitor printout has two lines running across it. The top line tracks your baby’s heart rate. The bottom line is the toco tracing, showing your contractions. At rest, the toco line sits at a low, relatively flat baseline. When a contraction begins, the line rises into a hill shape, then falls back down as the contraction ends.
You can estimate the spacing of your contractions by looking at the distance between the peaks of those hills. The paper feeds through the machine at a set speed, with time markings along the edge, so each small box represents a specific interval. Your nurse uses this pattern to track how your labor is evolving over hours, not just in isolated moments.
Why Placement Matters
The toco works by detecting the shape change in your abdomen when the uterus contracts. That means it needs to sit right over the top of the uterus, where the outward push is strongest. If the sensor slips too far to one side or too low, the tracing can flatten out and miss contractions entirely, even though you’re clearly feeling them. This is one of the most common reasons for a “lost signal” during labor, and it’s usually fixed by simply repositioning the belt.
Your position matters too. Rolling onto your side, getting up to use the bathroom, or leaning forward on a birth ball can all shift the sensor. It’s normal for the nurse to come in and readjust multiple times. If you’re moving frequently during labor (which is encouraged for comfort), expect some interruptions in the tracing.
Accuracy and Body Size
One significant limitation of the toco is that it becomes less reliable as maternal body size increases. A study published in Reproductive Sciences found that patients classified as obese were 82% less likely to have their contractions detected by the toco compared to nonobese patients. Among obese women in the study, contractions went undetected in roughly 76% of cases, compared to about 36% of nonobese patients.
The reason is straightforward: the toco depends on detecting a subtle shift in the shape of the abdominal wall. More tissue between the uterus and the sensor dampens that signal. The same issue can arise earlier in pregnancy, when the uterus is still small and doesn’t create as much outward displacement during a contraction. If your care team is having trouble picking up contractions they suspect are happening, they may rely more on your own reports of what you’re feeling, or move to internal monitoring if the clinical situation calls for it.
When You’ll Encounter a Toco
You’ll most commonly have a toco placed during labor and delivery, where it stays on for continuous or intermittent monitoring depending on your hospital’s protocols and your risk factors. But it also shows up earlier in pregnancy. If you go to the hospital with possible preterm contractions, the toco is the first tool used to check whether your uterus is actually contracting and how frequently. It’s also part of routine monitoring during a non-stress test in the third trimester, where the goal is to observe the baby’s heart rate patterns in relation to any uterine activity.
In all of these scenarios, the toco serves the same basic purpose: giving your care team a real-time, continuous view of your contraction pattern without any invasive procedure. It’s one of the simplest and most widely used tools in obstetric care, and understanding what it does (and what it doesn’t do) can make the experience of being hooked up to a monitor feel a lot less mysterious.