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, also known as CTG, uses ultrasound to measure your baby’s heart rate. At the same time it also measures the contractions in the uterus. CTG is used both antenatally (before birth) 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 baby’s heart is placed on the abdomen (tummy) 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 uses ultrasound to measure the baby’s heart rate. The other measures the pressure in the abdomen and the mother’s contractions. Your midwife may put some jelly between the plate and your abdomen 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.
A thin tube called a catheter may also be inserted through the vagina into your uterus. The tube measures the strength of contractions. You may feel some discomfort when the catheter is inserted but it should not be too painful.
How does cardiotocography work?
Cardiotocography uses ultrasound to detect the baby’s heart rate. Ultrasound is a high-frequency sound that you cannot hear, but it can be emitted and detected by special machines.
Ultrasound travels freely through fluid and soft tissues. However, ultrasound is reflected back (it bounces back as ‘echoes’) when it hits a more solid (dense) surface. For example, the ultrasound will travel freely though 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 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?
CTG monitors your baby’s heartbeat and its response to your contractions. It can be used if:
- Your baby is coming early or seems smaller than expected.
- You have high blood pressure.
- You are expecting twins.
- The baby has passed meconium (opened its bowels) into the amniotic fluid.
- The midwife thinks there may be a problem having listened with a Pinard or Doppler machine.
- You have an epidural for pain relief.
- You have labour speeded up with Syntocinon®.
Are there any side-effects or complications from cardiotocography?
CTG does not use any radiation; it is considered a very safe test. If you have external CTG you are unlikely to have any side-effects. Internal monitoring usually involves putting in a catheter. Catheters are generally used with no problems but, in a few cases, they may cause a urinary infection. Speak with your doctor if you have any symptoms of a urinary infection which include:
- Pain on passing urine.
- Increased need to pass urine.
- Smelly or cloudy urine.
- Fever or temperature.
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