Cardiotocography can be used to monitor a baby's heart rate and a mother's contractions while the baby is in the uterus.
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.
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 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, 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 device called an electrode is inserted through the vagina and placed on the baby's scalp. This device records the heart rate.
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 (dense) 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 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.
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?
It is normal for a baby's heart rate to vary between 110 and 160 beats a minute. This is much faster than your own heart rate, which is about 60-100 beats per minute. A heart rate in your baby that doesn't vary or is too low or too high may signal a problem.
Changes in the baby's heart rate that occur along with contractions form a pattern. Certain changes in this pattern may suggest a problem. If test results suggest your baby has a problem, your doctor may decide to deliver the baby right away. This may mean you need to have a caesarean section or an assisted delivery using forceps.
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.
If you are having a CTG and the trace remains normal over 20 minutes, it will usually be removed.
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. If CTG is used when it is not necessary, it may be that it increases the chances of having interventions (such as forceps or caesarean section) that weren't really needed. Women who have infections such as herpes, hepatitis B or C or HIV do not usually have internal monitoring, as it may increase the chance of passing the infection on to the baby.
Further reading and references
Intrapartum care for healthy women and babies; NICE Guideline (Dec 2014, updated Feb 2017)
Intrapartum care; NICE Quality Standard, December 2015
Alfirevic Z, Devane D, Gyte GM; Continuous cardiotocography (CTG) as a form of electronic fetal monitoring (EFM) for fetal assessment during labour. Cochrane Database Syst Rev. 2013 May 315:CD006066. doi: 10.1002/14651858.CD006066.pub2.
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.