An external fetal monitor is a common tool used during pregnancy and labor to assess the health of the fetus and track the labor process. This device, often called a cardiotocograph, uses two sensors placed on the mother’s abdomen to provide continuous, real-time data. The monitor creates a visual record of the baby’s heart activity and the mother’s uterine contractions, allowing clinicians to interpret how the fetus is tolerating the labor environment. This monitoring is especially helpful in high-risk pregnancies or during labor inductions, as contractions temporarily reduce blood flow to the placenta. By providing this feedback, the electronic fetal monitor offers an important window into the dynamic process of childbirth.
Understanding the Monitoring Strip Setup
The data from the external fetal monitor is displayed on a screen or printed onto a specialized paper strip, which is divided into two distinct sections. The top tracing is dedicated to the Fetal Heart Rate (FHR), while the bottom tracing records the Uterine Activity (UA), also known as contractions. Both tracings share the horizontal axis (X-axis), which represents time moving forward. Bold vertical lines typically separate one-minute intervals, with lighter lines marking 10-second increments within each minute.
The vertical axis (Y-axis) is different for each tracing, representing its specific measurement scale. On the upper FHR tracing, the Y-axis measures the fetal heart rate in beats per minute (BPM). The FHR is detected by an ultrasound transducer placed on the maternal abdomen. The lower UA tracing’s Y-axis represents the relative strength of the contractions, captured by a pressure-sensitive device called a tocodynamometer (Toco), which rests on the mother’s abdomen.
Interpreting Contraction Characteristics
The lower tracing shows contractions as upward peaks, with the line returning to a baseline between each peak. Three distinct characteristics are evaluated when examining these peaks: frequency, duration, and intensity.
Frequency
Frequency measures how often contractions occur. It is calculated by measuring the time from the beginning of one contraction peak to the beginning of the next. For example, if the space between the start of two successive peaks spans three one-minute blocks on the X-axis, the contraction frequency is every three minutes.
Duration
Duration measures the length of a single contraction. This is found by measuring the time from the point where the line begins to rise to the point where it returns to the baseline. In active labor, contractions typically last between 60 and 90 seconds. Normal labor progression often involves contractions occurring every two to five minutes.
Intensity
Intensity is represented by the height or amplitude of the peak on the external monitor. The external Toco only provides a relative measure of intensity, which is subjective and easily affected by device placement, maternal body type, or movement. It does not accurately reflect the true internal pressure of the uterus. Only an Internal Uterine Pressure Catheter (IUPC), placed inside the uterus, can provide an objective, quantitative measurement of contraction strength in mmHg.
Assessing Fetal Response to Contractions
To understand how the fetus is handling labor, the upper FHR tracing must be analyzed in correlation with the lower UA tracing. The first step is to establish the FHR baseline, which is the average heart rate of the fetus over a 10-minute period, excluding any temporary rate changes. A normal baseline heart rate for a term fetus ranges from 110 to 160 beats per minute.
Variability and Accelerations
Variability appears as small, irregular fluctuations in the baseline heart rate. Moderate variability, defined as fluctuations between 6 and 25 BPM, is a reassuring indicator that the fetal nervous system is functioning properly. The tracing should also be checked for accelerations, which are temporary, abrupt increases in the FHR above the baseline. These increases are often associated with fetal movement and are considered a sign of a well-oxygenated fetus.
Decelerations
Decelerations are temporary drops in the fetal heart rate classified based on their timing relative to the contraction peak. An early deceleration is a gradual drop in FHR that mirrors the contraction, with the lowest point aligning with the peak of the contraction. This is generally considered a benign finding caused by head compression. In contrast, a late deceleration is a gradual drop that begins after the contraction peak and returns to baseline only after the contraction is over. This pattern is often a sign of reduced oxygen flow to the fetus and typically requires immediate clinical attention.