Electronic Fetal Monitoring (EFM), often referred to as Cardiotocography (CTG), is a widely used technology during labor to continuously track a baby’s physiological response to the stress of contractions. This monitoring system produces a continuous, real-time visual record for the medical team to analyze. The primary goal of reading this tracing is to assess the fetus’s oxygenation status and determine if the baby is tolerating the labor process well. By systematically evaluating the patterns on the strip, clinicians can make informed decisions about managing the labor and ensuring the well-being of the baby throughout delivery.
The Two Tracks: Fetal Heart Rate and Uterine Contractions
The monitoring strip is divided into two distinct tracks, each presenting different physiological data simultaneously. The upper section of the strip is dedicated to the Fetal Heart Rate (FHR), which is measured in beats per minute (bpm). This line provides a continuous graph of the baby’s heart activity, with each small vertical square typically representing 10 bpm.
The lower track records the maternal Uterine Contractions (UCs), often called the tocogram. This line indicates the frequency and duration of the contractions. The paper strip moves continuously, with each large horizontal block generally representing one minute of time, establishing a clear temporal relationship between the baby’s heart rate and the mother’s uterine activity.
Interpreting the FHR trace requires comparing it directly to the timing of the UCs on the track below. The medical team analyzes how the baby’s heart rate reacts before, during, and after a contraction to determine the fetal response to the temporary reduction in blood flow that occurs with each uterine squeeze. Without examining both tracks together, it is impossible to accurately classify the FHR changes.
Defining the Fetal Heart Rate Baseline
The Fetal Heart Rate Baseline represents the average heart rate of the baby over a ten-minute period, excluding periods of marked changes, such as accelerations or decelerations. This baseline rate serves as the established resting heart rate for the fetus during labor. For a term fetus, a normal baseline rate is considered to be between 110 and 160 beats per minute.
A rate sustained above 160 bpm is classified as fetal tachycardia, which can be a response to various factors, including maternal fever, infection, or the baby’s attempt to compensate for chronic oxygen deprivation. Conversely, a sustained rate below 110 bpm is known as fetal bradycardia, which can indicate immediate concerns like cord compression.
The baseline is determined by the balance between the sympathetic and parasympathetic nervous systems. A change in the baseline, either too high or too low, often prompts the care team to investigate underlying causes, such as medications, infection, or a change in fetal status.
Interpreting Variability and Accelerations
Variability
Fetal heart rate variability refers to the slight, irregular fluctuations of the heart rate around the established baseline. This “jiggle” in the line is a positive sign, reflecting the continuous interaction between the sympathetic and parasympathetic branches of the autonomic nervous system. A healthy, well-oxygenated fetal brain is necessary to produce this beat-to-beat irregularity.
Variability is quantified by the amplitude range of these fluctuations. Moderate variability, defined as an amplitude range of 6 to 25 bpm, is considered the most reassuring finding, as it reliably suggests the absence of significant metabolic acidosis. Minimal variability, with fluctuations of 5 bpm or less, may be caused by a temporary state like fetal sleep or maternal medication, but it can also signal potential issues with fetal oxygenation.
Absent variability, where no fluctuations are detectable, is a significant concern. Marked variability, with an amplitude greater than 25 bpm, is less common and sometimes associated with acute events like cord compression.
Accelerations
Accelerations are temporary, abrupt increases in the fetal heart rate above the baseline, typically occurring in response to fetal movement or external stimulation. In a fetus at 32 weeks gestation or later, an acceleration is defined as an increase of at least 15 bpm above the baseline, lasting for at least 15 seconds.
The presence of accelerations is highly reassuring, acting as a clear indicator of a healthy, responsive nervous system. They demonstrate that the fetus has sufficient oxygen reserves and is physiologically well-prepared to handle the demands of labor. Accelerations may occur spontaneously or in association with a uterine contraction.
Understanding Decelerations: When the Heart Rate Drops
Decelerations are temporary decreases in the FHR below the baseline, and they are classified based on their shape, depth, and, most importantly, their timing relative to the uterine contraction. The systematic classification of these dips helps the care team understand the underlying physiological cause and the potential risk to the fetus. The three primary types are early, late, and variable decelerations.
Early decelerations appear as a smooth, symmetrical dip in the FHR that “mirrors” the uterine contraction, meaning the lowest point of the deceleration (nadir) happens at the same time as the peak of the contraction. These are typically considered benign and are caused by compression of the fetal head during a contraction, which triggers a reflex slowing of the heart rate through the vagus nerve. They do not usually indicate a problem with fetal oxygenation.
Late decelerations are also smooth and symmetrical, but their timing is distinct: they begin after the contraction has started and their nadir occurs after the peak of the contraction, with the FHR returning to the baseline only after the contraction has ended. This pattern is often concerning because it is associated with uteroplacental insufficiency, meaning there is a temporary reduction in blood flow and oxygen transfer to the fetus during the contraction. Causes can include maternal hypotension or placental issues, requiring prompt intervention to improve fetal oxygen delivery.
Variable decelerations are the most common type and are characterized by an abrupt, jagged, and often irregular shape, frequently described as V, W, or U-shaped. They are named “variable” because their onset, depth, and duration vary with successive contractions, and they may occur with or without an accompanying contraction. These are typically caused by compression of the umbilical cord, which momentarily restricts blood flow.
While isolated variable decelerations with moderate variability are often tolerated, repetitive or severe variable decelerations that drop below 70 bpm or last longer than 60 seconds may signal significant cord compression and reduced oxygenation. The medical team considers the entire context of the strip—the baseline, the variability, and the type of decelerations—to determine if the patterns are reassuring, indeterminate, or abnormal, guiding necessary actions.