Cardiotocography (CTG) is a medical technique used during pregnancy and labor to monitor the fetal heart rate (FHR) and uterine contractions (UC). This non-invasive electronic fetal monitoring provides a continuous, graphical record to assess the well-being of the fetus. By tracking how the baby’s heart rate responds to contractions, the CTG helps determine if the baby is receiving sufficient oxygen during the later stages of pregnancy and throughout labor.
The Cardiotocography Procedure
The most common method is external monitoring, achieved using two specialized transducers secured to the mother’s abdomen. One transducer utilizes Doppler ultrasound technology to detect fetal heart movements and record the heart rate pattern. The second transducer, a pressure-sensitive device called a tocodynamometer, is placed near the top of the uterus to measure the frequency and duration of uterine contractions. This external approach is non-invasive and is the standard for most monitoring sessions.
In certain circumstances, internal monitoring may be employed during labor when the external signal is unclear or a more precise reading is required. This method requires the amniotic sac to be ruptured and the cervix to be sufficiently dilated. A fetal scalp electrode is gently attached to the baby’s scalp to obtain a direct and accurate electrical measurement of the heart rate. Additionally, an intrauterine pressure catheter (IUPC) can be inserted into the uterus to measure the actual strength of contractions, offering more detailed uterine activity data than the external tocodynamometer.
Indications for Fetal Monitoring
CTG monitoring is ordered to assess the fetus for potential distress or when certain risk factors are present. One common indication is a maternal report of reduced or absent fetal movement, which signals a need for immediate well-being assessment. The test is also routinely used for pregnancies that extend past the due date, as the placenta may become less efficient in providing oxygen and nutrients.
Continuous monitoring is often necessary during labor for various conditions:
- Maternal health conditions, including pre-eclampsia, chronic hypertension, or diabetes.
- Intrauterine growth restriction or issues with amniotic fluid volume, such as oligohydramnios.
- When labor is induced or augmented with medications like oxytocin.
- Bleeding or evidence of infection like maternal fever.
Interpreting the CTG Tracing
Interpreting the CTG tracing involves analyzing several specific parameters. The first component is the baseline fetal heart rate (FHR), which is the average rate maintained over a 10-minute period, excluding accelerations and decelerations. A normal baseline FHR typically falls within the range of 110 to 160 beats per minute (bpm). A rate consistently below 110 bpm is termed bradycardia, while a rate sustained above 160 bpm is called tachycardia.
Moderate variability represents the natural, irregular fluctuations in the FHR around the baseline. This moderate fluctuation is defined as having an amplitude between 6 and 25 bpm. Absent or minimal variability, where the fluctuation is 5 bpm or less, can be a concerning sign of fetal hypoxia.
Accelerations are temporary, abrupt increases in the heart rate above the baseline. The presence of accelerations demonstrates a robust fetal response to movement or contractions. Decelerations are temporary drops in the FHR and are categorized by their shape and timing in relation to uterine contractions.
Early decelerations are shallow, symmetrical drops that mirror the contraction, beginning and ending at the same time as the contraction, and are generally considered benign. Late decelerations are concerning because they begin after the peak of the contraction and return to baseline only after the contraction has ended, suggesting a delay in oxygen transfer to the baby. Variable decelerations appear as an abrupt, V-shaped drop and recovery, often varying in their timing with contractions, and are typically associated with umbilical cord compression. A prolonged deceleration is a single drop lasting two minutes or longer, requiring prompt clinical intervention.