Can CTG monitoring prevent HIE? The problem of prevalence.

Hypoxic ischaemic encephalopathy (HIE) is a condition seen in newborn infants who display signs of high blood acid levels and brain injury. Some recover, while for some infants HIE is the first sign of long-term neurological injury. It is uncommon, with reported prevalence rates of around 18 per 10,000 births (Hollowell et al., 2011).

Previous research

Only one of the randomised controlled trials comparing CTG use with intermittent auscultation reported on HIE as an outcome measure. That was the Athens based trial (Vintzileos et al., 1993), subsequently criticised for an apparent breach of the randomisation process (Keirse, 1994). This trial recruited 746 women in the CTG monitoring group, and 682 in the intermittent auscultation group. One baby in the CTG monitoring group developed HIE and two in the intermittent auscultation group. The difference was not significant.

The numbers of women recruited to this trial were too small to provide useful information about which form of fetal monitoring might be superior when it comes to preventing HIE. So we really don’t have good evidence to tell us anything very much about HIE. So when I spotted the title of this new study, “HIE following intrapartum asphyxia: Is it avoidable?”, I was curious to see what they were suggesting (Tarvonen et al., 2025).

What did they do?

This team of researchers, based in Finland, set out to assess the impact of low oxygen levels in labour (asphyxia) on the occurrence of HIE and to see whether any of the cases might potentially have been preventable. To do this, they looked at births at seven hospitals in Finland that occurred between 2005 and 2024, where both an admission CTG and then ongoing CTG monitoring was performed, and the baby was subsequently diagnosed with HIE. These babies all had blood tests soon after birth, testing for markers of low oxygen (pH, base excess, S100 beta, erythropoietin) and had their placentas examined histologically.

Because there is no current way to accurately measure low fetal oxygen levels during labour, they defined asphyxia as the presence of an umbilical artery pH of less than seven and / or base excess of less than or equal to -12 mmol/L; or a pH in the range of 7.0 – 7.09 and a base excess of -10 to – 11.9 mmol/L plus elevated levels of erythropoietin, S100 beta, an Apgar of less than four at five minutes of age, resuscitation beyond 10 minutes, any grade of HIE, or perinatal death.

What did they find?

First, they reviewed admission CTGs from 23,467 women who gave birth by a planned pre labour caesarean section at term. When the CTG was normal, no case of HIE was seen. Among the 1,219 women with an abnormal admission CTG, 13 gave birth to a baby with HIE (1.1%). They use this finding to state that a normal admission CTG therefore excludes the possibility of prelabour neurological injury. This is, of course, only true in the population they tested – not in women who planned to labour and give birth vaginally.

They identified 314 babies with a diagnosis of HIE, born among 317,126 women who did plan to labour (a prevalence of 10 per 10,000 births). Almost half the affected babies were born to women with no identifiable risk factors. Of these babies, 44.9% had a normal admission CTG. Babies with an abnormal admission CTG were more likely to have more severe HIE or to die, while babies with a normal admission CTG were more likely to have severe acidaemia at birth.

Next, they examined labour events that might have contributed to the outcome and the timeliness of responses to abnormal heart rate patterns. When the CTG was normal on admission (141 women), 106 had no placental abruption, cord prolapse, uterine rupture, or maternal cardiac arrest – events that are considered to be so catastrophic that even timely care might not prevent HIE. Of this 106 women, 57 gave birth by caesarean section or instrumental birth. The authors determined that this was done in a timely manner in 36 women (and they do include definitions of how they determined this to be the case in the methods section of the paper). Another 49 women gave birth vaginally more than 45 minutes after the onset of a pathological heart rate pattern. They therefore concluded that for 70 women (49.6% of those with HIE and a normal admission CTG, 0.02% of women who planned to labour, 22% of women with babies with HIE), HIE might potentially have been avoided if action was taken faster.

They didn’t explore whether there were potentially avoidable cases of HIE among the infants with an abnormal admission CTG.

Does this mean that CTGs are better than intermittent auscultation?

No it does not. This study excluded women who had intermittent auscultation used for all or part of labour, so it provides no evidence about whether intermittent auscultation might have performed better, worse, or the same as CTG use. The same heart rate patterns that define a CTG recording as normal or abnormal can also be detected by intermittent auscultation. It is therefore likely that (as long as good intermittent auscultation techniques were used) that the results would have been the same as with CTG use.

Why is prevalence a problem?

The authors of the paper conclude that CTGs play a critical role in screening for compromised fetuses, and that delayed care might have played a role in the causation of HIE for some. This might be taken by some as proof that all women should be advised to have CTG monitoring. But let’s take a detour and look at the issue of prevalence to show why that might not be as logical an assumption as it appears to be.

The authors of this study did not provide a number for how many of the women who planned to labour had a normal or abnormal admission CTG. Nor do they provide information about the rate of CTG abnormalities during labour. Reynolds et al. (2022) have done this analysis. In their study, 60% of babies with HIE had a pathological (very abnormal) heart rate pattern, and 26% of healthy babies also had this pattern. I’ll use the 10 in 10,000 incidence rate for HIE reported by Tarvonen. Now let’s do the maths.

10,000 women plan to go into labour, and ten of these will have a baby with HIE. Six of them (60%) will have a pathological CTG in labour. Of the 9,990 women with healthy babies, 2,597 (26%) will also have a pathological CTG. In total, 2,603 women will have a pathological CTG, of whom 0.2% (six) will give birth to babies with HIE. Telling the difference between the 99.8% of babies who will be fine and the 0.2% who will not be fine is tricky.

If we also use Tarvonen’s figure of 22% of babies with HIE having a potentially preventable cause – we are now down to one baby (22% of six) for whom fetal heart rate monitoring might make a difference. Out of 10,000. That’s an awful lot of CTG monitoring for a tiny, and not even certain, benefit.

So in summary…

The paper by Tarvonen and colleagues reports on well designed research that provides useful new information. It doesn’t change my position on CTG use. Continuing to double down on the belief that if people were just a bit smarter and better at using CTGs and used them for more women then we would fix the problem IS the problem. We need an approach to fetal monitoring that really does improve outcomes for babies with few downsides for women and we won’t get there while we continue to do the same old things that haven’t worked for the past 60 years.

[And because I know Susan Bewley is going to ask – no the Tarvonen study didn’t discuss timing of cord severance, and yes it is possible that early cord severance might be what led to the ischaemia that caused the injury for some babies.]

Sign Up for the BirthSmallTalk Newsletter and Stay Informed!

Want to stay up-to-date with the latest research and course offers? Our monthly newsletter is here to keep you in the loop.

By subscribing to the newsletter, you’ll gain exclusive access to:

Exciting Announcements: Be the first to know about upcoming courses. Stay ahead of the curve and grab your spot before anyone else!
Exclusive Offers and Discounts: As a valued subscriber, you’ll receive special discounts and offers on courses. Don’t miss the chance to save money while investing in your knowledge development.

Join the growing community of BirthSmallTalk folks by signing up for the newsletter today!

Sign up to the Newsletter

Join Now

References

Hollowell, J., Puddicombe, D., Rowe, R., Linsell, L., Hardy, P., Stewart, M., Redshaw, M., Newburn, M., McCourt, C., Sandall, J., Macfarlane, A., Silverton, L., Brocklehurst, P., & Birthplace in England Collaborative Group. (2011). The Birthplace national prospective cohort study: perinatal and maternal outcomes by planned place of birth Birthplace in England research programme. http://openaccess.city.ac.uk/3650/1/Birthplace_Clinical_Report_SDO_FR4_08-1604-140_V03.pdf

Keirse, M. (1994, Jun 01). Electronic monitoring: Who needs a Trojan horse? Birth, 21(2), 111-113. https://doi.org/10.1111/j.1523-536X.1994.tb00246.x

Reynolds, A. J., Murray, M. L., Geary, M. P., Ater, S. B., & Hayes, B. C. (2022, Jun). Fetal heart rate patterns in labor and the risk of neonatal encephalopathy: A case control study. European Journal of Obstetrics & Gynecology and Reproductive Biology, 273, 69-74. https://doi.org/10.1016/j.ejogrb.2022.04.021

Tarvonen, M., Jernman, R., Stefanovic, V., Tuppurainen, V., Karikoski, R., Haataja, L., & Andersson, S. (2025, May 8). Hypoxic-ischemic encephalopathy following intrapartum asphyxia: is it avoidable? American Journal of Obstetrics & Gynecology, in press. https://doi.org/10.1016/j.ajog.2025.04.073

Vintzileos, A. M., Antsaklis, A., Varvarigos, I., Papas, C., Sofatzis, I., & Montgomery, J. T. (1993, Jun 01). A randomized trial of intrapartum electronic fetal heart rate monitoring versus intermittent auscultation. Obstetrics & Gynecology, 81(6), 899-907. https://www.ncbi.nlm.nih.gov/pubmed/8497353