Continuous electronic fetal monitoring uses a printout, often called a tracing, to provide a simultaneous view of the baby’s heart rate and the mother’s uterine activity. This dual-graph record is a real-time visualization of the labor process, allowing clinicians to assess how the uterus is contracting and how the baby is responding. The bottom section of this printed strip contains a wavy line that maps the contractions, showing their characteristics over time. Understanding the measurements and patterns represented by this line translates the physical force of labor into clinical data.
How Contraction Data is Collected
The data that creates the contraction waveform is gathered using one of two primary methods, which affects how the resulting measurement must be interpreted. The most common approach is external monitoring, which uses a device called a tocodynamometer, or toco. This pressure-sensitive sensor is secured to the mother’s abdomen over the uterine fundus. The toco records the frequency and duration of contractions, but it can only measure relative intensity; a stronger contraction creates a higher peak than a weaker one on the same patient.
For a precise measurement of intensity, an Intrauterine Pressure Catheter (IUPC) is required, which is a form of internal monitoring. This thin, flexible tube is inserted through the cervix into the uterine cavity, where it measures the actual fluid pressure. The IUPC provides an objective reading of the contraction’s strength in millimeters of mercury (mmHg). This internal method offers a measurement of uterine work but can only be used once the membranes have ruptured.
Reading the Tracing Paper Scales
The tracing paper is a grid that serves as a common reference for both time and pressure. The horizontal axis of the lower tracing represents time, moving from right to left as the paper is continuously fed through the monitor. The paper moves at a standard speed, often 3 centimeters per minute, which is the basis for all time calculations.
Darker vertical lines on the grid mark one-minute intervals, while the lighter vertical lines between them represent ten-second segments. This consistent grid allows for simple calculation of how often and how long contractions are occurring. The vertical axis on the contraction tracing measures pressure, marked with a scale ranging from 0 to 100 or 120. When using an IUPC, this scale directly corresponds to pressure in mmHg, providing an absolute measure of the force generated by the uterus.
Analyzing the Contraction Waveform
Interpreting the contraction waveform involves measuring four specific characteristics of the wave-like shape. The duration of a contraction is the time from the point where the line begins to rise (the increment) to the point where it returns to the resting pressure (the decrement). This is measured horizontally across the grid by counting the number of ten-second boxes the contraction spans. A typical contraction during active labor may last between 60 and 90 seconds.
Frequency is determined by measuring the time interval from the beginning of one contraction to the beginning of the next. This is a horizontal measurement across the grid and is expressed in minutes, such as “contractions every three minutes.” The intensity or amplitude of the contraction is the height of the peak, known as the acme, measured vertically against the pressure scale.
When an IUPC is in place, the intensity is a quantitative reading in mmHg, with active labor contractions often generating 40 to 60 mmHg. If only an external toco is used, the amplitude is a relative measure, not an absolute force. The final characteristic is the resting tone, which is the pressure recorded on the line between contractions. Maintaining a low resting tone, typically less than 20 mmHg, is necessary to ensure adequate blood flow and oxygenation to the placenta and the baby between uterine squeezes.
Identifying Different Labor Patterns
The overall pattern of the waveforms provides a clinical picture of the labor’s progress and efficiency. A tracing showing weak, irregular, and low-amplitude contractions, perhaps registering between 5 and 25 mmHg, is often characteristic of Braxton Hicks contractions or very early, latent labor. As labor progresses, the pattern shifts to increasing consistency, duration, and intensity.
Active labor is characterized by a regular pattern of strong, long-lasting contractions occurring with greater frequency, such as every two to three minutes. A healthy pattern requires uterine relaxation, demonstrated by the line returning to a low resting tone between each contraction. A pattern known as tachysystole, or hyperstimulation, is a concerning finding where contractions occur too frequently, defined as more than five contractions in a ten-minute window averaged over 30 minutes. This high frequency reduces the time the uterus has to rest, potentially compromising the oxygen supply to the baby.