Uterine contractions are the physiological driving force behind labor and delivery. These contractions thin and open the cervix (dilation) and push the fetus through the birth canal. Measuring contraction characteristics—frequency, duration, and intensity—is necessary for managing labor. Assessing the strength helps healthcare providers determine if labor is progressing effectively, ensures fetal well-being, and guides intervention decisions.
Self-Assessment: How Intensity Feels
A person’s subjective experience is the first way contraction intensity is evaluated. Patients use descriptive language like “mild,” “moderate,” or “strong” to communicate the perceived level of discomfort and muscle tightening, helping providers understand pain progression.
In a clinical setting, healthcare providers supplement patient reports by manually palpating the uterus during a contraction. They place their hands on the abdomen to feel the firmness of the uterine wall at its peak. A mild contraction feels soft and is easily indented by the fingers.
A moderate contraction feels firmer and is difficult to indent but remains pliable. A strong contraction makes the abdominal wall feel very firm and impossible to indent. While subjective, this hands-on assessment provides a relative measure correlated with the patient’s ability to cope, such as needing to pause conversation or change breathing patterns.
External Monitoring of Contraction Timing
The most common method of electronically monitoring contractions uses an external device called a tocodynamometer (toco). This non-invasive, pressure-sensing plate is secured to the abdomen with a belt, typically placed over the uterine fundus. The toco detects the change in tension across the abdominal wall as the uterus shortens and hardens during a contraction.
The tracing produced by the toco accurately measures the frequency and duration of contractions. However, it does not measure the actual internal pressure or true intensity. The reading is relative, meaning a higher peak indicates a stronger contraction relative to others in that patient’s tracing.
Accuracy is easily affected by factors like belt tightness, patient position, or maternal adipose tissue. The device may record a low number even during a painful contraction if the transducer is positioned incorrectly or if the patient shifts. Therefore, the toco is primarily a tool for timing and pattern recognition, requiring manual palpation to estimate true strength.
Clinical Measurement of True Intensity
For a precise, quantitative measurement of contraction intensity, clinicians rely on the Intrauterine Pressure Catheter (IUPC). This invasive device is the gold standard for directly measuring the force generated by the uterus. The IUPC is a thin, flexible catheter inserted through the cervix and into the amniotic space after the membranes have ruptured.
The catheter contains a pressure transducer that directly measures the pressure inside the uterus in millimeters of mercury (mmHg). This direct measurement provides an objective number for contraction strength, unlike the external monitor’s relative reading. The IUPC is typically used when labor is not progressing or when external monitoring is unreliable.
The ultimate clinical measure of uterine work is the calculation of Montevideo Units (MVUs). This value is derived from IUPC readings and quantifies the total intensity of contractions over a ten-minute period. To calculate MVUs, the baseline resting tone of the uterus (typically 8 to 12 mmHg) is subtracted from the peak pressure of each contraction.
The resulting net pressure values for all contractions within a ten-minute window are summed together. For example, if four contractions occur with net pressures of 40, 50, 45, and 55 mmHg, the total MVU value would be 190. A range of 200 to 250 MVUs is considered adequate contractile activity for normal progression during active labor.