Cardiotocography (CTG) is the process of monitoring a fetal heartbeat and the contractions of the uterus during pregnancy. CTG usually helps during the third trimester. It helps monitor fetal well-being and detects symptoms of fetal distress at an early stage.
Fetal monitoring has been used for a long time now. Even before the 1970s, people used fetal monitoring, which was done by people listening to the baby through a stethoscope designed especially for pregnancy (Source). It was called a fetoscope. Fetal monitoring helps analyze how the baby is coping with labor.
Today we use electronic fetal monitoring, which provides a graph to see how the baby’s heart rate responds during the contractions.
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How to read contractions on fetal monitor?
Fetal monitors today can be used internally or externally, continuously or intermittently. According to The American College of Obstetricians and Gynecologists (ACOG), women with low-risk get fetal monitoring done through an electric fetal monitor, a stethoscope, or a handheld doppler.
Electric fetal monitors display the fetal heart rate and the mother’s contractions either on a computer monitor or a paper graph. The fetal heart rate is usually displayed on the top of the monitor screen, whereas the contractions are displayed on the bottom.
On a graph paper, the fetal heart rate is displayed on the left and contractions are displayed on the right.
The y-axis of the fetal heart rate graph displays the beats per minute (bpm), which are measured in increments of ten and markings of every 30 beats. On the x-axis of the fetal heart rate graph, markings display a span of every one minute with lighter lines within those minutes that display a ten-second increment.
The graph of the contractions displays millimeters of mercury (mmHg) that is supposed to measure the strength of the contractions. The higher the number, the stronger the contraction.
How to read a contraction monitor?
To interpret a contraction monitor or CTG, you can learn the acronym DR C BRAVADO. It is very popular and makes it easier for you if you’re wondering how to read contractions on a monitor and understand the various characteristics of a CTG or a contraction monitor.
DR means Define Risk. It evaluates if the pregnancy is high or low risk.
Some of the factors involved in making a pregnancy high-risk are listed below.
Maternal medical illness
- Gestational diabetes
- Post-date gestation
- Multiple gestations
- Previous C-section
- Intrauterine growth restriction
- Premature rupture of membranes
- Congenital malformations
- Oxytocin induction
- Augmentation of labor
- Drug abuse
- Absence of prenatal care
C is for the number of contractions occurred in ten minutes. There are two things that you need to analyze about every contraction.
Duration: How long did a contraction last?
Intensity: How strong was the contraction?
Individual contractions are represented as peaks on the CTG monitoring uterine activity.
BRA represents the Baseline Rate of the fetal heart. It is the average heart rate of the fetus in a period of ten minutes.
Normally, a fetal heart rate is between 110-160 beats per minute.
If the baseline rate of the fetal heart is greater than 160 bpm, then it is called Fetal Tachycardia.
- Fetal hypoxia
- Fetal or maternal anemia
- Fetal tachyarrhythmia
- Chorioamnionitis – maternal fever included
If the baseline rate of the fetal heart is less than 100 bpm for three minutes or more, then it is called Fetal Bradycardia. Although it is normal to have a baseline rate of the fetal heart between 100-120 bpm in the situations listed below:
- Post gestation
- Occiput posterior
- Transverse presentations
If the baseline rate of the fetal heart is less than 80 bpm for more than three minutes, it indicates severe prolonged bradycardia, which could be a symptom of severe hypoxia.
- Cord prolapse
- Prolonged cord compression
- Epidural and spinal anesthesia
- Maternal seizures
- Rapid fetal descent
If these factors can’t be dealt with, immediate delivery is recommended.
V is for Variability, which refers to baseline variability. It assesses the variation of fetal heart rate from one beat to the next. The interaction between the nervous system, chemoreceptors, baroreceptors, and cardiac responsiveness causes this variability.
Variability of fetal heart rate in response to the change in environment is good because a healthy fetus always adapts to the changes around him. Normal variability of the fetal heart rate is between 5-25 bpm. You can calculate the variability by calculating the deviation of peaks and troughs from the baseline rate (in bpm).
Categories of variability
- Reassuring: 5 – 25 bpm
- Less than 5 bpm for 30 – 35 minutes
- More than 25 bpm for 15 – 25 minutes
- Less than 5 bpm for more than 50 minutes
- More than 25 bpm for more than 25 minutes
Factors involved in reduced variability
- Fetal sleeping – doesn’t last longer than 40 minutes
- Fetal acidosis – due to hypoxia and more likely to happen in case of late decelerations
- Fetal tachycardia
- Drugs – opiates, benzodiazepines, methyldopa, magnesium sulfate
- Prematurity – reduced variability at earlier gestation (<28 weeks)
- Congenital heart abnormalities
A is for Accelerations. Accelerations are a sudden increase in the baseline fetal heart rate, which is greater than 15 bpm for more than 15 seconds. Accelerations occur in response to the uterine contractions which a sign of a healthy fetus.
D denotes Decelerations. Decelerations are a sudden decrease in the baseline fetal heart rate, which is greater than 15 bpm for more than 15 seconds. The autonomic and somatic nervous system controls the fetal heart rate. Fetus reduces its heart rate to preserve myocardial oxygenation and perfusion, as a response to hypoxic stress. This reduction is known as deceleration.
5 different types of decelerations
1. Early Deceleration
Early deceleration starts as the uterine contraction starts and recovers as uterine contraction stops. It occurs due to increased fetal intracranial pressure, which causes the vagal tone to increase. Early decelerations are considered to be physiological instead of pathological.
2. Variable Deceleration
Variable decelerations occur as a rapid fall in the baseline fetal heart rate with a variable recovery phase (Source). They have no link to the uterine contractions. They are commonly observed during labor and in patients who have reduced amniotic fluid volume.
Compression in the umbilical cord usually leads to variable decelerations in the baseline fetal heart rate. They are sometimes resolved if the mother changes her position.
3. Late Deceleration
Late decelerations are observed at the peak of the uterine contraction, and they recover as the contraction stops. It also indicates the insufficient flow of blood to the uterus and placenta, and this decrease in blood flow causes fetal hypoxia and acidosis.
Causes of reduced uteroplacental blood flow
- Maternal hypertension
- Uterine hyperstimulation
In case of late decelerations, fetal blood sampling for pH is recommended. If fetal blood pH is acidotic, it indicates fetal hypoxia and requires immediate C-section.
4. Prolonged Deceleration
A prolonged deceleration lasts for more than three minutes.
- Non-reassuring: if it lasts between 2-3 minutes
- Abnormal: if it lasts for more than three minutes
5. Sinusoidal Pattern
This type is rare but very critical. The sinusoidal pattern is associated with high rates of fetal morbidity and mortality.
Characteristics of a sinusoidal CTG pattern
- A regular, smooth, wave-like pattern
- Frequency of 2-5 cycles per minute
- Stable baseline rate of 120-160 bpm
- Lack of beat to beat variability
Sinusoidal pattern indicates
- Severe fetal hypoxia
- Severe fetal anemia
- Fetal/maternal hemorrhage
The sinusoidal pattern indicates an immediate need for C-section, and results can be poor if ignored or dealt with carelessly.
O refers to Overall Impression. After the CTG assessment, you need to determine your overall impression, which is described as the following:
- Baseline heart rate: 110-160 bpm
- Baseline variability: 5-25 bpm
- None or early
- Variable decelerations with noncritical characteristic for less than 90 minutes
- Baseline heart rate:
- 100-109 bpm
- 161-180 bpm
- Less than 5 for 30-50 minutes
- More than 25 for 15-25 minutes
- Variable decelerations with noncritical characteristic for 90 minutes or more
- Variable decelerations with any critical characteristic and up to 50% of contractions for 30 minutes or more
- Later decelerations with 50% of contractions for less than 30 minutes, and no fetal clinical risk factors like vaginal bleeding or meconium
- Baseline heart rate:
- Below 100 bpm
- Above 180 bpm
- Baseline heart rate:
- Baseline variability:
- Less than 5 for more than 50 minutes
- More than 25 for more than 25 minutes
- Variable decelerations with any critical characteristic and up to 50% of contractions for 30 minutes or less if any maternal or fetal clinical risk is involved.
- Late decelerations for more than 30 minutes or less if any maternal or fetal clinical risk is involved.
- Acute bradycardia, or a single prolonged deceleration for three minutes or more.
Fetal monitors or CTGs help assess the fetal well-being, and the contractions during labor. It has quite a few benefits such as detection of fetal compromise, but sometimes a false-positive leads to unnecessary surgical intervention.
In 1958, the first Electronic Fetal heart rate Monitoring (EFM) was introduced at Yale University, and since than EFM has been widely used for the assessment of the fetus (Source). The National Center for Health Statistics of the USA reported the use of EFM in 755 cases per 1,000 live births in 1991. It’s widely used in many hospitals, especially during labor of high-risk patients.