A tocodynamometer (often shortened to “toco”) is a pressure-sensing device strapped to a pregnant person’s abdomen that detects uterine contractions by measuring how much the belly wall tightens and pushes outward. It doesn’t measure the actual force inside the uterus. Instead, it picks up the shape change on the surface of the abdomen each time the uterine muscle contracts, translating that movement into a waveform on a monitor strip.
The Guard-Ring Design
The core of a tocodynamometer is a strain gauge mounted inside a rigid ring, a design first developed by C.N. Smyth in the 1950s and still used in essentially the same form today. The rigid ring sits flat against the abdomen and acts as a frame of reference. In the center of that ring, a small pressure-sensitive button or piston protrudes slightly. When the uterus contracts, the abdominal wall firms up and pushes inward against that central button while the surrounding ring stays in place. That tiny difference in displacement is what the sensor actually measures.
The strain gauge converts this mechanical pressure into an electrical signal. As the piston is pushed inward, the strain gauge deforms very slightly, changing its electrical resistance in proportion to how much force is applied. The monitor reads that resistance change and plots it as a rising line on the screen or paper strip. When the contraction ends and the abdominal wall relaxes, the piston returns to its resting position and the tracing drops back toward baseline.
Where It’s Placed and Why
The sensor is positioned over the uterine fundus, the top of the uterus, because that’s where contractions generate the most outward tension against the abdominal wall. A nurse secures it with an elastic belt snug enough to maintain contact but loose enough to remain comfortable. The goal is to place the device where there is the least amount of soft tissue between the skin surface and the uterus, so the contraction signal comes through as clearly as possible.
Once the belt is in place, the nurse adjusts the tension so the monitor reads roughly 25 relative units above the initial baseline between contractions. Then the baseline is “zeroed” by pressing a reference button on the monitor while the uterus is relaxed. This calibration step ensures the resting tone registers near the bottom of the tracing, leaving room on the scale for contraction peaks to show clearly. If the person shifts position or the belt loosens, the baseline can drift and the nurse may need to reset it.
What the Tracing Shows (and Doesn’t Show)
The numbers on a toco tracing are relative units, not true pressure readings in millimeters of mercury (mmHg). The device tells you that a contraction happened, roughly how long it lasted, and how frequently contractions are coming. It cannot tell you how strong a contraction actually is in terms of intrauterine pressure. For that reason, tocodynamometry is considered a qualitative monitoring technique: it assesses changes in uterine shape rather than the actual force generated during a contraction.
Clinicians look at several features on the tracing. Contraction frequency is counted as the number of contractions within a 10-minute window, averaged over 30 minutes. In early labor, contractions typically come every 10 to 15 minutes. By active labor, that increases to 3 to 5 contractions per 10-minute window, and during the second stage of labor (pushing), frequency can reach 5 to 6 per 10 minutes. Six or more contractions in a 10-minute window is classified as tachysystole, a pattern that warrants close attention. Contraction duration also changes as labor progresses, starting at around 10 to 15 seconds early on and gradually lengthening to 40 to 45 seconds.
If clinicians need actual pressure measurements, they use an intrauterine pressure catheter (IUPC), a thin tube placed inside the uterus through the cervix. An IUPC provides quantitative readings in mmHg. During the first stage of labor, intrauterine pressure typically ranges from 25 to 50 mmHg per contraction, rising to 80 to 100 mmHg during the second stage, with a resting baseline tone of 8 to 12 mmHg between contractions. A toco cannot provide any of these numbers, but it avoids the small infection risk that comes with placing a catheter inside the uterus.
When the Signal Gets Unreliable
The biggest limitation of a tocodynamometer is that anything between the uterus and the sensor can muffle the signal. Maternal body mass index has a significant effect. One retrospective study found that obesity was the strongest independent risk factor for the toco failing to detect contractions, with obese patients roughly 82% less likely to have contractions picked up compared to patients with lower BMI. The extra adipose tissue absorbs the subtle shape change before it reaches the sensor surface.
Gestational age matters too. The same study found that evaluations in the mid-trimester (below about 25 weeks) were significantly less reliable, likely because the uterus is smaller and contractions are weaker at that stage. For patients who are obese or being evaluated early in pregnancy with symptoms of preterm labor, clinicians may need to rely on alternative monitoring methods rather than the toco alone.
Practical issues also degrade the signal. If the belt is too loose, the sensor loses contact. If it’s too tight, it can compress the abdomen and artificially raise the baseline. Movement, breathing, and even coughing can create artifacts on the tracing that look like small contractions but aren’t. Nurses routinely compare what they feel by hand (palpation) with what the toco shows to confirm the tracing is accurate.
How It Compares to Electrical Monitoring
A newer approach called electrohysterography (EHG) uses surface electrodes on the abdomen to pick up the electrical signals generated by the uterine muscle itself, similar to how an ECG detects heart muscle activity. While a toco responds to the mechanical consequence of a contraction (the belly getting harder), EHG detects the electrical depolarization of uterine muscle fibers that causes the contraction in the first place. This gives EHG the ability to measure contraction strength quantitatively and potentially detect contractions earlier in their development.
EHG is not yet standard in most labor and delivery units, but it addresses several toco limitations. Because it reads electrical activity rather than surface pressure, it is less affected by body composition. For now, though, the guard-ring tocodynamometer remains the default tool for noninvasive contraction monitoring during labor, largely because it is simple, inexpensive, and requires no special electrode preparation.