Understanding the lines on a fetal monitoring screen provides valuable insights into the progression of labor and the baby’s well-being. External fetal monitoring is a common, non-invasive technique used during pregnancy and labor to simultaneously track two critical measurements. The primary purpose of this electronic surveillance is to record the baby’s heart rate and the mother’s uterine activity, which includes contractions. Learning to interpret the tracing empowers a laboring person to better understand the information being gathered by the care team. This involves recognizing the separate components of the tracing and applying a systematic approach to reading the waves.
The Components of External Fetal Monitoring
The electronic fetal monitoring strip displays two distinct lines, each representing a separate physiological function. The upper line, typically generated by a Doppler ultrasound transducer, records the Fetal Heart Rate (FHR) in beats per minute. The lower line records Uterine Activity (UA) and is produced by a device called a tocodynamometer, often referred to as a “Toco.”
The Toco is a pressure sensor placed externally on the mother’s abdomen, usually secured over the uterine fundus. This device measures the tension and pressure changes on the abdominal wall as the uterine muscle tightens during a contraction. When a contraction occurs, the Toco registers the change in tension and translates it into a visible wave on the monitoring strip. It detects external pressure changes, not the fluid pressure inside the uterus.
How to Measure Contraction Strength and Timing
Interpreting the contraction line requires analyzing the peaks and valleys on the lower tracing to determine three key characteristics of uterine activity: frequency, duration, and relative strength.
Frequency
Frequency measures how often contractions occur by timing the interval from the start of one contraction’s peak to the start of the next. On the monitoring strip, time is marked by vertical lines, with darker lines often representing one-minute intervals. Clinicians count the number of contractions occurring within a 10-minute window, averaged over a 30-minute period, to establish the frequency.
Duration
Duration is the total length of time a single contraction lasts. This is measured from the point where the tracing line first begins to rise from the baseline to the point where it returns to the resting level. True labor contractions typically last between 30 and 70 seconds.
Relative Strength
Relative strength is visually represented by the height, or amplitude, of the contraction’s peak. The higher the peak, the stronger the contraction appears on the monitor. The tracing usually begins with the line resting at a baseline, which represents the uterine resting tone between contractions. A contraction appears as a wave, smoothly rising from the baseline, reaching its peak, and then gradually falling back down to the resting tone.
Reading Different Patterns of Uterine Activity
Applying these measurements helps differentiate various contraction patterns, most commonly distinguishing between non-labor and true labor contractions. Non-labor contractions, known as Braxton Hicks contractions, are typically irregular in their frequency and duration. On the monitor, these contractions often appear with low amplitude. They are sometimes alleviated or disappear entirely with a change in activity or position.
True labor contractions, by contrast, follow a more consistent and progressive pattern. They become regular, increase in amplitude, and decrease the interval between the onset of each contraction. In the active phase of labor, contractions typically last 30 to 70 seconds and occur every two to five minutes, with the Toco reading a higher relative strength.
Another pattern of uterine activity is tachysystole, which describes excessive contraction frequency. This is specifically defined as having more than five contractions in a 10-minute period, averaged over a 30-minute window. This pattern is important to identify because excessively frequent contractions can reduce the time the uterus has to relax, potentially leading to a temporary decrease in oxygen flow to the baby.
Limitations of External Contraction Monitoring
While the Toco provides valuable data, it is a non-invasive external device, and its readings have inherent limitations that must be considered. The external monitor is highly accurate for timing, recording the frequency and duration of contractions. However, the height of the tracing’s peak only indicates the relative strength of the contraction on the abdominal wall, not the true internal pressure or intensity within the uterus.
The Toco’s reading is significantly influenced by external factors that can distort the tracing. These factors include the tightness of the belt securing the monitor, the mother’s body position, and maternal size, which can make the signal unreliable. A weak-looking contraction on the monitor may, in reality, be quite strong if the Toco is poorly positioned or the belt is loose.
For this reason, the care team assesses the true intensity of a contraction through manual palpation, feeling the firmness of the abdomen during a contraction. The patient’s subjective feeling of pain or pressure is a more reliable indicator of true contraction strength than the Toco tracing’s amplitude. If the monitor tracing appears inadequate or concerning, or if the patient feels a strong, persistent contraction that does not register properly, staff should be alerted immediately.