Cardiotocography is usually called a 'CTG' by doctors and midwives. It can be used to monitor a baby's heart rate and a mother's contractions during labour.
Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
What is cardiotocography?
Cardiotocography (CTG) measures your baby's heart rate. At the same time it also monitors the contractions in the womb (uterus). CTG is used both before birth (antenatally) and during labour, to monitor the baby for any signs of distress. By looking at various different aspects of the baby's heart rate, doctors and midwives can see how the baby is coping.
The image below shows the equipment used to do a CTG. It normally stands by your bedside whilst you are being monitored.
By Steven Fruitsmaak, CC BY 3.0, via Wikimedia Commons
What happens during cardiotocography?
CTG is most commonly carried out externally. This means that the equipment used to monitor the baby's heart is placed on the tummy (abdomen) of the mother.
An elastic belt is placed around the mother's abdomen. It has two round, flat plates about the size of a tennis ball which make contact with the skin. One of these plates measures the baby's heart rate. The other assesses the pressure on the tummy. In this way it is able to show when each contraction happens and an estimate of how strong it is.
The midwife may put some jelly on the skin to help get a strong signal.
The CTG belt is connected to a machine which interprets the signal coming from the plates. The baby's heart rate can be heard as a beating or pulsing sound which the machine produces. Some mothers can find this distracting or worrying but it is possible to turn the volume down if the noise bothers you. The machine also provides a printout which shows the baby's heart rate over a certain length of time. It also shows how the heart rate changes with your contractions.
If you have CTG before you are in labour you may be asked to press a button on the machine every time the baby moves. At this time you will not be having any contractions so the CTG will only monitor the baby's heart rate.
Occasionally during labour, if a signal can't be found using the external monitor, or when monitoring is more important, internal monitoring can be used. For internal monitoring, a small, thin device called an electrode is used. This is inserted through the vagina and neck of the womb (which will be opening during labour) and placed on the baby's scalp. This device records the baby's heart rate.
If you have a twin (or higher multiples) pregnancy, internal monitoring can only be used on the baby closest to the neck of the womb.
Additionally internal monitoring can only be used when the baby is going to be delivered head first. Internal monitoring will not work on a breech (bottom or foot first) presentation.
How does cardiotocography work?
CTG uses sound waves called ultrasound to detect the baby's heart rate. Ultrasound is a high-frequency sound that you cannot hear but it can be sent out (emitted) and detected by special machines.
Ultrasound travels freely through fluid and soft tissues. However, ultrasound bounces back as 'echoes' (it is reflected back) when it hits a more solid surface. For example, the ultrasound will travel freely through blood in a heart chamber. But, when it hits a solid valve, a lot of the ultrasound echoes back. Another example is that when ultrasound travels though bile in a gallbladder it will echo back strongly if it hits a solid gallstone.
So, as ultrasound 'hits' different structures in the body, of different density, it sends back echoes of varying strength.
In external CTG monitoring, a special type of ultrasound, called Doppler®, is used. This type of ultrasound is used to measure structures that are moving, making it useful for monitoring heart rate.
Internal CTG monitoring is slightly different. This uses a wire electrode, attached to the baby's scalp, to detect the electrical signals from the baby's heartbeat, measuring the heart rate that way.
The other plate on the CTG measures how tense the mother's tummy (abdomen) is. This measurement is used to show when the uterus is contracting.
What can cardiotocography show?
CTG is used to detect signs that the baby is in distress - for example, if it's not getting enough oxygen.
Many different things can be recorded on a CTG; your baby's heart rate, the baby's movements and your contractions. These are all shown on the image below.
By LosHawlos, (GFDL) (CC-BY-SA), via Wikimedia Commons
Healthcare professionals look at different things on the CTG to get an overall picture of how the baby is doing. There are several different things for them to consider, including:
- The baby's heart rate. A normal heart rate for a baby during labour is between 110 to 160 beats a minute. If the baby's heart rate is persistently low or high, this can suggest a problem.
- The variability, or fluctuations, in the baby's heart rate. It's a good sign if the baby's heart rate changes slightly between beat-to-beat - this suggests that their brain is working well. If their heart rate stays very similar for a long time, this can suggest a problem.
- Patterns of heart rate changes in the baby, in response to contractions.
- The baby's heart rate increasing for a short time, and then going back to normal (called an acceleration) is a reassuring sign.
- The baby's heart rate can also decrease for short periods (called a deceleration). This is normal if it happens during the beginning of contraction, and goes back to normal straight after the contraction. However, if they are happening without contractions, happening late in a contraction or are lasting a long time after the contraction finishes, they can be a sign that the baby is in difficulty.
Healthcare professionals will use all of this information to interpret the CTG results. They will decide if the CTG results are:
- Reassuring - no signs of problems with the baby.
- Suspicious - some signs that the baby is in distress.
- Pathological - signs that strongly suggest the baby is in distress.
If the results are suspicious or pathological, clinicians will assess further and try to correct anything that could be affecting the baby. If this doesn't work, or if the CTG suggests that the baby is in serious distress, they are likely to recommend delivering the baby straight away for its safety. This might be using forceps or a ventouse (vacuum cup), or via a Caesarean section.
What is cardiotocography used for?
In a normal, low-risk delivery, CTG is not usually needed. The midwife will listen to your baby's heart rate from time to time to check it is normal. However, in certain situations, continual monitoring with CTG is advised. These include:
- Your baby is coming early or seems smaller than expected.
- You have high blood pressure.
- You have a high temperature (fever).
- You have an infection.
- You pass fresh blood whilst in labour.
- You are expecting more than one baby (twins or more).
- The baby has opened its bowels (passed meconium) into the amniotic fluid.
- The midwife thinks there may be a problem, having listened with a Pinard® or Doppler® machine.
- If your membranes have ruptured more than 24 hours before your labour starts.
- If your baby is in an unusual position.
- You have labour speeded up with Syntocinon® or need an epidural for pain relief.
- If you have an epidural for pain relief during labour, CTG may be used for half an hour after an epidural has been put in, or after top-ups for the epidural.
Depending on the exact situation, some women may have continuous (ongoing) CTG monitoring. In other cases, women might have the CTG on for a short time only, and have it removed if the trace is normal.
Are there any side-effects or complications from cardiotocography?
CTG does not use any radiation; it is considered a very safe test. Depending on the machine used, it may prevent you from being able to move around freely during labour.
The aim of using CTG is to pick up signs that the baby is in trouble, allowing doctors and midwives to deliver the baby sooner, to prevent the baby from coming to harm.
However, like any test, CTG can sometimes be wrong. It can give 'false positives' - signs that the baby is in trouble, when it's actually not - leading to interventions (such as a forceps or Caesarean section) which are not really needed.
For this reason, CTG monitoring is usually not recommended for women who have no risk factors for birth complications, and no other signs that the baby is in trouble, as it's more likely to give false positives in this situation. It's a better test when used in women with higher-risk pregnancies.
Further reading and references
Intrapartum care for healthy women and babies; NICE Guideline (Dec 2014 - updated Dec 2022)
Intrapartum care; NICE Quality Standard, December 2015 - last updated February 2017
Alfirevic Z, Devane D, Gyte GM, et al; Continuous cardiotocography (CTG) as a form of electronic fetal monitoring (EFM) for fetal assessment during labour. Cochrane Database Syst Rev. 2017 Feb 32:CD006066. doi: 10.1002/14651858.CD006066.pub3.
Grivell RM, Alfirevic Z, Gyte GM, et al; Antenatal cardiotocography for fetal assessment. Cochrane Database Syst Rev. 2015 Sep 12(9):CD007863. doi: 10.1002/14651858.CD007863.pub4.