TOCO on a baby monitor stands for tocodynamometer, the sensor that tracks uterine contractions during pregnancy and labor. It’s one of two readings you’ll see on the fetal monitoring screen: the top line typically shows the baby’s heart rate, while the bottom line (labeled TOCO) shows when contractions are happening and how often they come. The sensor itself is a small, disc-shaped device strapped to the outside of the mother’s belly.
What the TOCO Sensor Actually Measures
The tocodynamometer is a pressure-sensitive strain gauge that detects changes in abdominal tension. When the uterus contracts, it tightens and pushes outward against the abdomen. The TOCO sensor picks up that change in shape and converts it into a waveform on the monitor. Each hill or peak on the TOCO tracing represents one contraction, and the valleys between peaks represent the resting periods in between.
The sensor reliably captures two things: how often contractions are happening (frequency) and roughly how long each one lasts (duration). What it cannot measure is how strong the contractions actually are. The height of the peaks on the screen doesn’t correspond to contraction intensity in any standardized way. A nurse will typically press on the top of the uterus by hand to gauge whether a contraction feels mild, moderate, or strong. Stronger contractions do tend to produce taller waveforms relative to weaker ones on the same patient, but the numbers aren’t comparable from one person to another.
Where the Sensor Gets Placed
The TOCO transducer is positioned on the abdomen over the top of the uterus (the fundus), wherever it feels firmest. When the baby is head-down, that firmest spot is usually over the baby’s buttocks near the top of the belly. An elastic belt holds it in place. If the sensor shifts during movement or repositioning, the tracing can become unreliable, so nurses may need to adjust it periodically. Between contractions, the monitor is typically zeroed out to a baseline reading of about 10 on the lower portion of the tracing.
How TOCO Works With the Heart Rate Monitor
The TOCO reading is only half the picture. The full monitoring setup, called cardiotocography, pairs contraction data with a continuous reading of the baby’s heart rate. What matters most to medical staff is the relationship between these two lines. They’re looking at how the baby’s heart rate responds each time the uterus contracts, because that relationship reflects how well the baby is getting oxygen.
A healthy pattern shows the baby’s heart rate staying stable or dipping slightly and recovering quickly during contractions. When the heart rate drops in sync with the peak of a contraction and bounces back promptly, that’s generally considered a normal response to brief pressure on the baby’s head. More concerning is when the heart rate drops after a contraction peaks and is slow to recover. This delayed pattern can signal that blood flow through the placenta is being temporarily reduced during contractions, and the baby is taking longer to recover from each one.
Sometimes the heart rate drops sharply and unpredictably, without a clear connection to contractions. This pattern often indicates pressure on the umbilical cord and tends to look different each time it happens. The TOCO tracing provides the crucial timing reference that lets staff distinguish between these patterns.
What the Numbers on the Screen Mean
Contraction frequency is measured from the start of one contraction to the start of the next. During active labor, contractions typically come every two to five minutes. Duration is measured from when a contraction begins to when it ends, usually lasting 45 to 90 seconds in active labor.
One specific pattern staff watch for is when contractions come too frequently. The clinical threshold is more than five contractions in a 10-minute window, averaged over 30 minutes. This pattern, called tachysystole, can reduce the rest time the baby needs between contractions to recover normal oxygen levels. When the TOCO tracing shows this pattern, the medical team will typically intervene to slow things down.
When the TOCO Sensor Is Less Reliable
The TOCO works by detecting changes in the contour of the abdomen, which means anything that makes those changes harder to detect will reduce its accuracy. The most significant factor is body weight. Research has found that patients classified as obese by BMI were 82% less likely to have their contractions detected by the external sensor. Greater abdominal girth means the contracting uterus produces a smaller relative change in the surface of the belly, giving the sensor less signal to work with.
Earlier gestational age also reduces accuracy. A smaller uterus in the mid-trimester may not displace the abdominal wall enough for the sensor to register contractions consistently. Movement, breathing, and even laughing can create noise on the tracing that looks similar to small contractions, making interpretation harder. If external monitoring isn’t capturing contractions reliably, medical staff may switch to an internal pressure catheter placed directly inside the uterus, which measures both frequency and true intensity of contractions. This option is only available once the membranes have ruptured.
What You’ll See on the Printout
The monitor produces a continuous paper printout (or digital equivalent) with two parallel tracings. The upper tracing records the baby’s heart rate, scaled in beats per minute. The lower tracing is the TOCO line, showing contraction activity. A flat baseline with periodic smooth hills indicates regular contractions with adequate rest in between. The tracing scrolls at a set speed, so medical staff can measure timing directly from the paper by counting the grid squares between peaks.
If you’re watching the monitor during labor, the TOCO number will rise as a contraction builds, peak, and then fall back toward baseline as the contraction ends. This rise and fall will often match what you feel physically, though the sensor sometimes registers contractions a moment before or after you notice them yourself. Occasionally the number creeps up from belt tightness or repositioning rather than an actual contraction, which is why nurses assess the tracing in context rather than reacting to any single spike.