CTG stands for cardiotocography, a monitoring technique used during pregnancy and labor to track a baby’s heart rate and the mother’s contractions at the same time. It’s one of the most common procedures in obstetric care, and if you’re pregnant or preparing for labor, there’s a good chance you’ll encounter it. The test gives doctors and midwives a real-time picture of how the baby is responding to contractions and whether the baby is getting enough oxygen.
What CTG Actually Measures
A CTG recording captures two things simultaneously. The first is the fetal heart rate, which normally sits between 110 and 160 beats per minute. The second is uterine activity: how often contractions are happening, how long they last, and the time between them. By looking at both together on a printed strip or screen, clinicians can see how the baby’s heart responds each time the uterus contracts. A healthy baby’s heart rate fluctuates naturally, speeding up and slowing down in small ways. When those fluctuations disappear or the heart rate drops in certain patterns during contractions, it can signal that the baby is under stress.
How the Monitor Works
Most CTG monitoring is done externally. Two sensors are strapped around your abdomen with elastic belts. One sensor uses Doppler ultrasound, the same technology used to detect movement, to pick up the baby’s heartbeat. The other is a pressure-sensitive device called a tocodynamometer. It sits flat against the skin and estimates the internal pressure of contractions by measuring how much the abdominal wall pushes against it.
In some situations, internal monitoring is used instead. This involves placing a small wire electrode directly on the baby’s scalp (or whichever part is closest to the cervix) to get a more precise heart rate reading. A thin tube called an intrauterine pressure catheter can also be inserted to measure contractions from inside the uterus. Internal monitoring is only possible after your water has broken, and it’s typically reserved for cases where external monitoring isn’t giving a clear enough signal or when closer surveillance is needed.
When CTG Is Used
CTG is used most often during the third trimester and throughout labor. In low-risk pregnancies, intermittent monitoring (checking the heart rate periodically rather than continuously) is common. Continuous CTG is more likely when there are risk factors that increase the chance of complications, such as preeclampsia, gestational diabetes, concerns about the baby’s growth, or signs during labor that the baby may not be tolerating contractions well. It can also be used as a “non-stress test” before labor begins, where the baby’s heart rate is monitored for 20 to 40 minutes to check on wellbeing without any contractions being induced.
Reading a CTG Strip
A CTG produces a continuous paper printout (or digital display) with two lines. The top line traces the fetal heart rate, and the bottom line shows uterine contractions. Clinicians assess several features of that tracing.
Baseline Heart Rate
This is the average heart rate over a 10-minute window. A normal baseline falls between 110 and 160 beats per minute. A rate consistently above or below that range can indicate problems like infection, oxygen deprivation, or other stressors.
Variability
Variability refers to the small, beat-to-beat fluctuations in heart rate, and it’s one of the most important things clinicians look at. These fluctuations reflect a healthy, active nervous system. Moderate variability, where the heart rate fluctuates by 6 to 25 beats per minute, is reassuring. Minimal variability (under 5 bpm) can occur when the baby is sleeping, but if it persists, it may suggest the baby isn’t getting enough oxygen. Absent variability, where the line is essentially flat, is a more serious concern. Marked variability, over 25 bpm, is unusual and warrants further evaluation.
Decelerations
Decelerations are temporary drops in the baby’s heart rate, and their timing relative to contractions is what matters most. Early decelerations mirror the contraction perfectly: the heart rate dips as the contraction peaks and recovers as it fades. These are generally harmless and caused by pressure on the baby’s head. Variable decelerations are sudden, sharp drops of 15 beats per minute or more that last at least 15 seconds. They look different from one contraction to the next and are often related to temporary compression of the umbilical cord.
Late decelerations are the most concerning pattern. The heart rate doesn’t start dropping until after the contraction has peaked, and it doesn’t recover until after the contraction has ended. This delayed response can indicate that the placenta isn’t delivering oxygen efficiently enough, and repeated late decelerations typically prompt closer monitoring or intervention.
What CTG Can and Cannot Tell You
CTG is essentially an indirect window into how the baby’s nervous system is functioning, reflected through its control of the heart rate. It’s good at identifying babies who are doing well: a reassuring CTG with a normal baseline, moderate variability, and no concerning decelerations is a strong sign of fetal wellbeing. Where CTG is less reliable is in predicting problems. An abnormal-looking tracing doesn’t always mean the baby is in danger. Babies sleep, move, and respond to medications the mother receives, all of which can temporarily change the tracing’s appearance. This is why clinicians interpret CTG in context, considering the full clinical picture rather than acting on a single reading in isolation.
CTG has been in widespread use since the early 1970s and remains a cornerstone of fetal monitoring. If your care team mentions putting you “on the monitor,” they’re almost certainly referring to a CTG.