Cardiotocography (CTG) is a standard, non-invasive procedure used during late pregnancy and throughout labor. This method provides healthcare providers with a continuous, real-time assessment of the fetus’s condition. By monitoring physiological data, CTG determines how the fetus is tolerating the intrauterine environment and the stresses of uterine contractions. It is widely used to ensure fetal well-being and identify potential signs of distress requiring timely intervention.
Cardiotocography: What the Acronym Means
CTG stands for Cardiotocography, a medical term derived from the Greek words “cardia” (heart), “tokos” (birth or labor), and “graphy” (to write or record). This technique is a combined electronic fetal monitoring system. Cardiotocography simultaneously tracks two parameters: the fetal heart rate (FHR) and uterine contractions (UC).
The core purpose of this dual measurement is to evaluate the relationship between the fetal heart pattern and the uterus’s activity. This combined assessment provides a more comprehensive picture than either measurement alone. Observing how the fetal heart responds during and after a contraction offers insight into the fetus’s oxygenation status and overall health. Monitoring both elements together screens for signs of fetal hypoxia (lack of adequate oxygen supply).
The Monitoring Procedure
The most common application is external monitoring, performed by placing two specialized devices, called transducers, on the mother’s abdomen. Elastic belts hold these transducers securely in place, connecting them to the cardiotocograph machine. One transducer uses Doppler ultrasound technology to continuously detect the fetal heart’s electrical signals, providing FHR data.
The second transducer, a tocodynamometer, is positioned near the top of the uterus. This pressure sensor records the frequency and duration of uterine contractions by measuring tension changes in the abdominal wall. The procedure requires the patient to lie down or sit semi-recumbent for the test’s duration, which often lasts between 20 minutes to several hours, depending on the clinical context.
If external monitoring does not provide a clear signal, internal monitoring may be used, but only after the amniotic sac has ruptured. Internal FHR monitoring involves attaching a small electrode directly to the fetal scalp via the cervix. Internal contraction monitoring uses an intrauterine pressure catheter (IUPC) inserted into the uterus to measure the actual strength of contractions, offering a more precise reading than the external tocodynamometer.
Understanding the CTG Tracing
The data collected is printed onto a continuous paper strip or displayed on a screen, creating the CTG tracing. The top line displays the fetal heart rate (FHR) in beats per minute, while the bottom line records uterine contractions. Providers analyze several features on this tracing to determine fetal well-being, beginning with the baseline FHR.
The baseline rate is the average FHR over a stable period, typically 110 to 160 bpm in a term fetus. A sustained rate below 110 bpm is bradycardia; above 160 bpm is tachycardia. Both can signal fetal stress. Another parameter is variability, the slight, beat-to-beat fluctuations around the baseline rate.
Normal variability is 5 to 25 bpm, indicating the fetal nervous system is functioning properly. Reduced variability (less than 5 bpm) can suggest a compromised fetus or fetal sleep. Providers also look for accelerations, temporary increases in FHR of at least 15 bpm lasting 15 seconds or more. The presence of accelerations is a reassuring sign that the fetus is healthy and well-oxygenated.
The most complex feature to interpret is the deceleration, a transient drop in FHR below the baseline. Decelerations are categorized by their shape, depth, and timing relative to the contraction.
Types of Decelerations
Early decelerations are usually shallow, mirroring the contraction with their lowest point (nadir) occurring at the contraction’s peak. They are often considered benign, resulting from head compression.
Late decelerations are more concerning because they begin after the contraction has peaked and return to the baseline only after the contraction has ended. This pattern is associated with placental insufficiency, meaning the fetus is not receiving adequate oxygen during the contraction. Variable decelerations appear as abrupt, V-shaped drops in FHR unrelated to the contraction timing. These are caused by temporary compression of the umbilical cord, and their frequency and depth determine the degree of concern.
Clinical Applications
CTG is used in two primary clinical settings: the antenatal period and during active labor.
Antenatal CTG (Non-Stress Test)
Antenatal CTG, performed before labor, is often referred to as a Non-Stress Test (NST) and monitors high-risk pregnancies. This test is ordered when there are concerns such as:
- Decreased fetal movement.
- Post-term pregnancy extending past 40 weeks.
- Maternal health conditions like hypertension or diabetes.
The goal of antenatal monitoring is to ensure the fetus remains reactive and well-oxygenated in the absence of contractions. An NST is considered reactive—a sign of fetal well-being—if the fetus exhibits an appropriate number of accelerations within a 20-minute period. If the test is non-reactive, further investigation may be required.
Intrapartum CTG
Intrapartum CTG is continuous monitoring performed once labor has started, when the fetus is subjected to the physiological stress of regular uterine contractions. This application is important when labor is induced or augmented with medications like oxytocin, or if a pre-existing maternal or fetal risk factor exists. By tracking the FHR response to contractions, healthcare teams make informed decisions about labor progression and determine if intervention is necessary to protect the fetus.