Cardiotocography (CTG) is a medical technique used during late pregnancy and throughout labor to assess the well-being of the fetus. This procedure simultaneously monitors two functions: the baby’s heart rate pattern and the mother’s uterine contractions. CTG’s primary function is to identify potential signs of fetal distress, particularly those related to a lack of oxygen (hypoxia). By providing a continuous record of these measurements, healthcare providers gain insight into how the baby is coping inside the uterus, especially during the dynamic process of labor. This monitoring is standard practice in many maternity settings to guide clinical decisions and promote a safer birth experience.
The Mechanics of Cardiotocography
The CTG procedure begins with placing two sensors, called transducers, directly onto the mother’s abdomen. One transducer uses Doppler ultrasound technology to detect the fetal heart rate, typically positioned over the area where the heartbeat is strongest. The second sensor, known as a tocodynamometer or “toco,” is placed over the top of the uterus (fundus) to record the frequency and duration of uterine contractions. This external device measures tension changes on the abdominal wall, indirectly tracking contraction timing.
Data from both transducers is sent to the cardiotocograph machine, which prints a continuous graph showing the fetal heart rate on the upper line and contractions on the lower line.
Internal Monitoring
If external monitoring fails to provide a clear or consistent signal, or when closer surveillance is required, internal monitoring may be used. The fetal heart rate can be tracked more directly using a Fetal Scalp Electrode (FSE), a small wire attached to the baby’s scalp through the cervix. To measure the true strength of contractions, an Intrauterine Pressure Catheter (IUPC) can be inserted into the uterus after the membranes have ruptured. Internal monitoring provides more accurate data, as it is less affected by factors like maternal movement or body mass.
Clinical Reasons for CTG Monitoring
Healthcare providers initiate CTG monitoring when there is an increased likelihood that the baby may experience challenges with oxygen supply. Before labor, CTG is often used as part of a non-stress test (NST) if the mother reports a decrease in fetal movement, checking for a reassuring heart rate response. Continuous CTG monitoring is also implemented for high-risk pregnancies due to pre-existing maternal conditions or complications that arise during the pregnancy.
High-risk conditions include:
- Gestational diabetes.
- High blood pressure (preeclampsia).
- Pregnancy extending past the due date (post-term).
- Labor induced using medication.
- Maternal complications, such as vaginal bleeding or fever.
Once labor begins, CTG serves as a surveillance tool to detect early signs of fetal hypoxia. This allows the medical team to intervene promptly if the baby appears stressed, assessing the baby’s physiological response to the demands of uterine contractions.
Interpreting the CTG Tracing
The interpretation of a CTG tracing is a systematic process that involves analyzing four main components of the fetal heart rate pattern in the context of the recorded uterine contractions.
Baseline Fetal Heart Rate (FHR)
This is the average heart rate measured over a 10-minute segment, excluding periods of accelerations, decelerations, or marked variability. A normal baseline rate for a full-term fetus is between 110 and 160 beats per minute (bpm). A baseline consistently below 110 bpm is termed bradycardia, while a rate above 160 bpm is called tachycardia. Both rates can be signs that the baby is struggling.
Variability
Variability describes the slight, irregular fluctuations in the baseline heart rate. Normal variability is between 6 and 25 bpm and is a strong indicator of a healthy, well-oxygenated fetal nervous system. When variability drops below 5 bpm, it can signal that the baby is either in a quiet sleep state or experiencing a lack of oxygen.
Accelerations
Accelerations are transient increases in the FHR above the baseline, defined as an increase of 15 bpm or more lasting at least 15 seconds. The presence of accelerations is a reassuring sign, often coinciding with fetal movement, suggesting the baby is well-oxygenated and active. The absence of accelerations on an otherwise normal tracing does not always indicate a problem.
Decelerations
Decelerations are temporary drops in the fetal heart rate below the baseline. Early decelerations are considered benign, as they mirror the shape of the contraction and are caused by head compression. Late decelerations are more concerning because the heart rate drop begins after the peak of the contraction, associating them with reduced placental blood flow and oxygen transfer. Variable decelerations are abrupt drops that vary in timing and shape relative to contractions, usually caused by umbilical cord compression. The severity and recurrence of these decelerations, especially late or severe variable types, correlate directly with the risk of fetal compromise.
What Happens After the Test
Once the CTG tracing is reviewed, the clinical team classifies the pattern to determine the next steps in care. A tracing showing a normal baseline rate, moderate variability, and accelerations or benign early decelerations is considered reassuring. In this scenario, monitoring may be discontinued, or labor continues with routine intermittent checks.
If the CTG trace is non-reassuring (e.g., reduced variability or concerning decelerations), immediate action is taken to improve the baby’s condition. Conservative interventions are the first line of management. These can include changing the mother’s position, such as turning her onto her side, to relieve potential compression on the umbilical cord or major blood vessels. Supplemental oxygen or intravenous fluids may also be given to improve blood flow to the placenta.
If the non-reassuring pattern persists despite these intrauterine resuscitation efforts, the situation is escalated for further assessment. This may involve performing a Fetal Scalp Blood Sample (FSBS) to measure the baby’s acid-base balance, providing a definitive assessment of oxygenation. If the CTG trace is acutely pathological, such as a prolonged, severe drop in heart rate, delivery may be expedited through an assisted vaginal birth or an emergency Caesarean section to ensure the baby’s immediate safety.