Cardiotocography (CTG): monitoring in pregnancy

By Cristiano Antonino On Jan 16, 2023

Cardiotocography (CTG) is the pregnancy monitoring test used to assess the health of the unborn child

Cardiotocography: during pregnancy, it is important to monitor the baby’s state of health constantly and frequently, in order to prevent or detect any problems in time

Cardiotocographic monitoring is a non-invasive test to which expectant mothers are subjected and which can be carried out from the 27th week if necessary, but more commonly it starts from the 37th week or in any case towards the last weeks of gestation.

The purpose of this check-up is to assess the well-being of the foetus and to record the frequency of any contractions of the mother, and thus the baby’s reactions.

It is also useful during labour to assess the status of the birth, whether it should be stimulated or whether a caesarean section is necessary, although doctors must be very careful of the risk of false positives.

What is cardiotocographic monitoring?

Cardiotocographic monitoring or cardiotocography is a non-invasive test that is used to check the well-being of the foetus (by monitoring its heart rate) but also to assess the level of any uterine contractions in progress for the mother-to-be.

The instrument used for this check is the cardiotocograph, with which the baby’s heartbeat is recorded.

Usually this test is repeated once a week until the onset of labour, and each session lasts at least half an hour, the time needed to assess the variations in the unborn baby’s heartbeat and to catch any sporadic contractions.

In order to understand if everything is going well, the baby must be awake: if he or she falls asleep and his or her heart rate is therefore more regular than in the waking state, you wait a few minutes, stimulating him or her with a massage, or you give the mother a sugary drink.

How is cardiotocographic monitoring done?

Cardiotocographic monitoring is done by having the mother-to-be lie down or sit up, and two probes connected to the cardiotocograph are placed on her abdomen.

One of these probes is ultrasound and is used to detect the fetal heartbeat: this instrument translates the variations in the heartbeat into a graph that is printed out on paper.

The second probe, on the other hand, is used to measure the intensity and frequency of the mother’s uterine contractions and is applied at the bottom of the uterus with a band: the sensor registers the variations in pressure in the mother’s abdominal wall caused by the contractions and here too the data is translated into a graph printed by the machine.

The mother feels no pain, the test is non-invasive and the result is virtually instantaneous and printed on paper in real time.

What is measured during cardiotocography?

During cardiotocography, doctors obtain information on the well-being of the unborn child, given not so much by the instantaneous measurement of its heartbeat, but by observing changes in the heart rate over time.

If the baby has a regular heartbeat, then there are probably no complications.

Conversely, if his heart rate is not variable (short-term variability and within a normal range), then there may be a problem.

Cardiotocographic monitoring can also tell a lot from the heartbeat’s reactions to the uterine contractions: if at contractions that the mother also feels, the baby’s heart rate undergoes abrupt decelerations, it may mean that the unborn baby is stressed, which is normal but only if labour is at an end.

If the baby is in this condition from the beginning of labour, it is necessary to intervene or at least investigate the reason.

Parameters obtained from cardiotocography

From cardiotocographic monitoring, the gynaecologist and obstetrician assess five parameters:

the baseline, i.e. the average basal heart rate (HRF)
the variability, i.e. the difference between maximum and minimum frequency (10 – 15 beats per minute)
the presence of accelerations, i.e. the increase in average heart rate;
the presence of decelerations, i.e. the sharp decrease in mean heart rate;
active fetal movements (MAF), i.e. the presence of fetal movements perceived by the mother.

If the result of the test is doubtful, there are suspicious elements or parameters that are not in the normal range, the gynaecologist may prescribe further investigations and tests such as ultrasound or Doppler flowmetry, which allows the correct exchange of oxygen between the placenta and the foetus to be verified.

False positives detected during the test

Cardiotocography during labour is the norm and indeed there is no mother who does not undergo this check before giving birth; however, it has a low specificity and this makes the analysis and reading of the chart more complex.

In fact, in many cases, the presence of abnormalities in the monitoring trace generates false positives, i.e. abnormalities that do not in fact correspond to foetal distress.

This is why, in cases of doubt, gynaecologists and midwives should proceed with further and different tests to investigate the actual state of health of the baby.

The interpretation of the cardiotocographic monitoring trace is in fact very complex: doctors must take into account not only the data they are analysing at that moment, but also the medical history of the mother-to-be, supplementing it with other instrumental tests, such as obstetric ultrasound.

This allows them to have a complete picture of the baby’s state of health and to avoid invasive interventions such as a caesarean section when in reality it is not necessary.

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