The visible record of labor contractions appears on a printout called a Cardiotocograph (CTG), which is a continuous electronic recording monitoring both the mother’s uterine activity and the baby’s heart rate simultaneously. This tracing helps healthcare providers assess how the baby is tolerating the stress of the contractions. The CTG is organized into two primary graphs: the upper section displays the fetal heart rate (FHR) in beats per minute, and the lower section charts the uterine contractions. The contraction pattern is represented by a wavy line in the lower graph, illustrating the onset, peak, and relaxation phase of each contraction.
How Uterine Activity is Recorded
The most common method for externally recording contractions is through a device called a Tocodynamometer, often referred to as a Toco. This external transducer is secured to the mother’s abdomen, typically over the top of the uterus, using a belt. The Toco is pressure-sensitive and detects the tightening and relaxing of the abdominal wall as the uterus contracts. This pressure change is then converted into the tracing line seen on the lower graph of the CTG paper.
The Toco measures only external pressure changes on the abdomen, not the true internal pressure or intensity within the uterus. Since the reading is affected by factors like the mother’s position and movement, it accurately measures the timing of contractions but only approximates their actual strength. For a direct measurement of true contraction intensity, an invasive device called an Intrauterine Pressure Catheter (IUPC) is used. The IUPC is a thin tube inserted into the uterus after the amniotic sac has ruptured, measuring pressure in millimeters of mercury (mmHg). The external Toco remains the primary, non-invasive monitoring method, despite the IUPC giving a definitive reading of contraction strength.
Understanding the Toco Tracing Grid
The paper on which the contractions are printed is a specialized grid, providing a standardized scale for interpretation. Contractions are plotted on the lower portion of the strip, where the vertical axis (Y-axis) represents pressure units. Although the Toco does not measure true internal pressure, this axis is typically marked with a scale ranging from 0 to 100 mmHg. The resting tone, the pressure between contractions, establishes the baseline, and contraction peaks rise above this level.
The horizontal axis (X-axis) represents time, which is necessary for calculating both the duration and frequency of the contractions. This axis is marked by fine and thicker lines, where each small box represents a 10-second increment. The thicker vertical lines mark the passage of one minute, meaning there are six 10-second boxes within each minute-long section. This structured grid allows healthcare providers to precisely measure the contraction pattern by counting the boxes the wave encompasses.
Interpreting Frequency, Duration, and Intensity
The visual interpretation of the contraction tracing focuses on three characteristics: duration, frequency, and a visual estimation of intensity.
Duration
Duration is measured by counting the 10-second boxes on the X-axis from the moment the line begins its upward slope (onset) until it returns to the established resting tone (end). For example, a contraction lasting 60 seconds will visually span six 10-second boxes on the tracing paper.
Frequency
Frequency is calculated by measuring the time interval from the start of one contraction to the start of the next, using the X-axis time markings. This measurement is typically expressed as the number of contractions occurring within a ten-minute window. During active labor, a typical frequency is approximately three to five contractions in a ten-minute period.
Intensity
Intensity is visually estimated by the height of the peak on the Y-axis. While the Toco reading is not a precise measure of internal pressure, a mild contraction shows a low peak, a moderate contraction reaches a mid-level peak, and a strong contraction peaks at a higher level. The tracing must show the line returning completely to the established baseline between contractions. This indicates the uterus is fully relaxing and allowing blood flow to the baby.