TOCO on an NST is the contraction monitor. Short for tocodynamometer, it’s a pressure-sensitive sensor placed on your belly that detects when your uterus tightens. While the other sensor on a non-stress test tracks your baby’s heart rate, the TOCO specifically measures uterine activity, showing the timing and frequency of contractions on a paper strip or screen.
How the TOCO Sensor Works
The TOCO is a small, disc-shaped device held against your abdomen with an elastic belt. It sits near the fundus, which is the top of your uterus, because that’s where contractions produce the most outward pressure. When your uterus contracts, your abdominal wall shifts slightly, and the sensor picks up that change in pressure. It then sends the signal to the monitor, which prints it as a wave on the lower line of your NST strip.
The upper line on the strip shows fetal heart rate. The lower line is the TOCO reading. Together, these two tracings let your care team see how your baby’s heart rate responds to contractions or movement, which is the whole point of the non-stress test.
What the Numbers Mean
TOCO readings are displayed in mmHg (millimeters of mercury), a standard unit of pressure. The numbers give you a rough sense of what’s happening, but they come with a major caveat: the TOCO reliably shows when contractions happen and how often, but it does not accurately measure how strong they are. The absolute number on the screen depends heavily on how tightly the belt is strapped, where the sensor sits, and your body type.
That said, general ranges are still useful for understanding what you’re seeing. When your uterus is at rest between contractions, the baseline usually hovers in the low range. Braxton Hicks contractions typically register between about 5 and 25 mmHg. True labor contractions in the active phase tend to fall between 40 and 60 mmHg, and during the pushing stage, readings can reach 50 to 80 mmHg. These numbers are approximate, and two people in identical labor can show very different TOCO values based on sensor placement alone.
Telling Braxton Hicks From Real Contractions
On the TOCO strip, Braxton Hicks contractions appear as small, irregular bumps. They don’t follow a pattern, and the peaks are relatively low. True labor contractions look different: the waves are taller, more uniform in shape, and they come at regular, shortening intervals. Your care team looks at the spacing and regularity of these peaks more than the height, precisely because the TOCO can’t reliably gauge intensity.
If you’re being monitored during a non-stress test in the third trimester and the TOCO picks up tightening you didn’t even feel, that’s common. Mild uterine activity is normal and doesn’t necessarily mean labor is starting. What matters most on an NST is how your baby’s heart rate behaves in response to that activity.
Why the TOCO Sometimes Misses Contractions
The TOCO works by detecting changes in the shape of your abdomen, which means anything that makes those changes harder to detect will reduce accuracy. The biggest factor is body mass index. A study published in Reproductive Sciences found that women classified as obese were 82% less likely to have contractions detected by the TOCO compared to non-obese women. Among 37 obese patients in that study, contractions went undetected in about 76% of cases, versus 36% in non-obese patients.
The sensor also loses accuracy when you shift position, cough, or tense your abdominal muscles. If you move and the TOCO slides even slightly off the fundus, the tracing can flatten out or show erratic spikes that don’t correspond to real contractions. Nurses will often reposition the sensor during monitoring to keep the signal clean. Earlier in pregnancy, when the uterus is smaller, contractions may not produce enough change in abdominal contour for the TOCO to register at all.
External TOCO vs. Internal Monitoring
The TOCO used during an NST is an external monitor, meaning it sits on top of your skin. It’s completely noninvasive, which is why it’s the standard tool for routine monitoring. But when precise contraction strength matters, particularly during a complicated labor, providers sometimes switch to an internal uterine pressure catheter (IUPC). This is a thin, fluid-filled tube placed inside the uterus alongside the baby, and it directly measures the pressure generated by each contraction.
An IUPC provides quantitative data on contraction intensity that the external TOCO simply cannot. It isn’t affected by body size, sensor placement, or movement. However, it requires ruptured membranes and carries a small risk of infection, so it’s reserved for situations where knowing exact contraction strength will change clinical decisions. For a standard non-stress test, the external TOCO provides all the information needed.
What You’ll Experience During the Test
A non-stress test is straightforward. You’ll recline in a chair or on a bed, and a nurse will strap two sensors to your belly: one for the baby’s heart rate (usually placed lower, over the baby’s chest) and the TOCO near the top of your uterus. The test typically runs for 20 to 40 minutes. You won’t feel anything from the sensors themselves, though the belts can feel snug.
During the test, you might be asked to press a button each time you feel the baby move. The monitor records everything simultaneously, so your care team can see whether the baby’s heart rate rises with movement (a reassuring sign called a “reactive” result). The TOCO tracing runs along the bottom, quietly documenting any uterine activity in the background. If the strip looks good, you’re typically done in under half an hour.