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A short guide on how Cardiotocography (CTG) is used in fetal monitoring

data curve CTG

This week, I have received questions about why there is so little research being done on children who experience oxygen deprivation during childbirth.

In my dissertation (I defended it in 2007), a large Swedish study was included where we compared two methods of identifying fetuses at risk of oxygen deprivation.

We compared the ability to measure lactate/pH in the fetal blood*. It has been quite some time since that study was conducted, but it is still relevant. Therefore, in this post and perhaps in future ones, I will discuss how we monitor the fetus during childbirth in the best possible way.

*10.1136/bmj.39553.406991.25. Epub 2008 May 25

Cardiotocography (CTG)

In 1820, what we now call the "wooden funnel" was invented, which provided the ability to listen to the fetal heartbeat and variations during childbirth through a funnel.

In 1870, a researcher named Schwartz described fetal sounds for the first time, saying they sounded like ringing bells (beautiful, isn't it?).

In 1958, the first studies on electrocardiography (ECG) were conducted, which involved learning to record electrical impulses from the fetal heart. Cardiotocography (CTG) was widely introduced in clinics by someone named Hon in 1977. CTG is a kind of ultrasound technique that measures the mechanical activity of the fetal heart.

The goal of fetal monitoring is to identify the fetuses that are experiencing oxygen deprivation and need to be delivered. We use this method during childbirth, during pregnancy, and sometimes in other special situations.

However, we must not forget that CTG provides a snapshot. A normal curve at one point in time may not be normal a couple of hours later if you understand what I mean. This sometimes creates a false sense of security.

When interpreting a CTG, it is important to consider all the available information about the fetus and the mother. This means taking into account the woman's medical history, previous pregnancies, deliveries, and the current pregnancy.

That's why we often ask these questions that may sometimes seem irrelevant. This is done to obtain the most accurate picture possible of the fetus's situation and to determine if any necessary actions need to be taken.

I have previously described that fetuses experiencing oxygen deprivation use defense mechanisms that allow ongoing oxygen deprivation to occur without causing permanent damage.

Therefore, it is important to be able to interpret signals from fetuses undergoing stress during childbirth. If we can do that, we can take appropriate actions when the reserves of the fetus are reduced and thus minimize the risk of serious harm to the fetus. It's all about our knowledge.


High sensitivity

The strength of the CTG method is that it can identify fetuses that are normally oxygenated, meaning they are healthy and normal, with relatively high certainty. It is also easier to identify the fetuses that should be delivered immediately. The method has what we call high sensitivity.

Low specificity

However, we know that CTG patterns deviate from the normal in about half of all deliveries. This means that half of all CTG curves interpreted as abnormal actually reflect a healthy and well-being baby. Tricky, isn't it? This further means that the method is quite poor in this regard. It has what we call low specificity.

Supplementary methods

Therefore, complementary methods such as something called ST analysis (STAN) and scalp blood sampling (pH/lactate) are now used to increase specificity and reduce the risk of unnecessary interventions. Otherwise, there is a risk of performing unnecessary procedures on healthy mothers and babies, which also carries risks.

When should we monitor, then?

The question I received earlier this week was why there is so little research being conducted, and indirectly, why don't we know more about this? When every year, children are still injured and perhaps even die during childbirth, even in Sweden.

Admission CTG

The discussion about admission CTG is sensitive. As you can understand, opinions vary. Recommendations and practices vary from country to country and sometimes even from clinic to clinic.

Several countries have now stopped using the method if the woman is assessed to have a low risk of complications, including a low risk of oxygen deprivation in the baby. This includes Norway, Denmark, the United Kingdom, Canada, Australia, and New Zealand. However, not in Sweden. At most clinics in Sweden, admission CTG is still performed today.

It is a kind of screening that lasts for about 20 minutes. Based on this recording, together with information about the woman and the baby, the risk can be assessed, and the further course of childbirth can be planned.

Normal CTG

With a normal CTG pattern, it can be reasonably assumed that the fetus is well-oxygenated. This means that the fetus likely has the capacity to handle a normal vaginal delivery. This also means that monitoring can often transition to just listening (with a wooden funnel/CTG machine) or intermittent CTG every 15 minutes.


However, if the CTG already shows an abnormal or pathological pattern during admission, childbirth should be monitored more closely.

Are we doing too much unnecessarily?

Now we come to what we recently discussed. How good is the predictive value of the method we use, really? Does this lead to us doing a lot of unnecessary interventions? On the other hand, does the absence of admission CTG lead to more harm to the babies?

In many places, it is believed that when CTG was introduced in modern obstetric care, it triggered a cascade of interventions. Interventions that threatened normal childbirth. We saw an increased proportion of cesarean sections and instrumental vaginal deliveries (vacuum extraction).


The Swedish Agency for Health Technology Assessment and Assessment of Social Services (SBU), an agency tasked with conducting independent evaluations of methods and interventions in healthcare, believes that a larger Swedish study is needed (surprise 😊).

The difficulty lies in the fact that CTG is now so integrated into Swedish obstetric care. From a research ethics perspective, it would be possible to randomly allocate women to an experimental group/a control group, but it would require a significant persuasion effort from both doctors, midwives, and perhaps even prospective parents. After all, we still believe in the method, don't we?

To be continued.

/Doctor Eva


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