Most of the time, when the fetal heart rate pattern is considered abnormal, there is no problem for the fetus. But sometimes there is, and earlier birth is probably a good idea when oxygen levels have fallen low enough to cause damage and the fetus can no longer adjust their system to cope. You can’t tell the difference between these two in the heart rate pattern though. In scientific terms, the positive predictive value of fetal heart rate monitoring is low.
Second line tests have been used almost for the entire duration of the history of CTG use, in the hope that these tests can provide information to separate the fetus who really would be better off if born quite soon from the fetus who is actually fine to wait out the course of labour. Fetal blood sampling is the most commonly used approach, where either pH (a measure of the overall acidity of the blood) or lactate (a specific acid) is measured. There’s a bunch of assumptions that sit behind this idea that may or may not actually hold up to scrutiny (like the idea that fetal scalp lactate levels reflect fetal oxygenation rather than the woman’s uterine muscle lactate levels).
There’s not actually a huge pile of research to support the use of fetal blood sampling in practice (which is not really a surprise given the state of research relating to all things about fetal monitoring!). There’s never been a randomised controlled trial showing whether using fetal blood sampling or not leads to better outcomes for the baby with an acceptable rate of intervention and complications of the woman. There’s also not a lot of research that establishes where to draw the line in the sand in terms of when to take action. So I’m always pleased when I see new research that tackles some aspect of fetal blood sampling, like the new paper from Godhard and colleagues (2025) from the UK. They set out to determine how accurate fetal blood sampling for detecting which babies would have a poor outcome.
What did they do?
The research team took information from electronic records relating to women who gave birth at one hospital between 2016 and 2017 and who had fetal blood sampling performed during their labour, no longer than an hour before the woman gave birth, and whose babies had an umbilical cord blood pH test after birth. Women with multiple pregnancies were excluded, and so were women with a prolonged fetal bradycardia before birth. They compared the pH result from the fetal blood sample with the babies Apgar scores at 5 minutes of age, and their umbilical cord pH. They used a cut off of 7.2 as their diagnosis of a low pH, in line with the 2017 NICE guideline definition. They didn’t look at clinically relevant outcomes like death, seizures, encephalopathy, or admission to the nursery – which I think is a weakness of this research. They do go on to mention they have plans for a larger study in the future that will include some of these outcomes.
What did they find?
In total, 407 women’s information formed the basis of this study – a relatively small sample size. Low pH levels (< 7.2) were present in 11.1% of the fetal blood samples. The authors didn’t give the kind of measures that I find most useful when giving information to women, but they did provide enough data to be able to calculate them, so I have done this for you. When the fetal blood pH level was less than 7.2, 74.4% of babies also had an umbilical cord pH of less than 7.2. This is the positive predictive value of fetal blood sampling in this study. The flip side of this is that the false positive rate (having a low fetal blood pH but the cord blood pH being 7.2 or more) was 25.6%. In other words, out of every four fetuses with a fetal scalp pH of under 7.2, three also had a low umbilical cord pH after birth and one did not.
Normal (7.2 or above) fetal blood pH levels performed only moderately well as predictors of normal cord blood pH levels. When the fetal blood pH was 7.2 or more, 40.6% of babies had a normal cord blood pH and 59.4% had a cord blood pH of under 7.2. Looking at Apgar scores, when the cord blood pH was under 7.2 then 17.2% of babies had an Apgar score of less than seven at five minutes of age. When the cord blood pH was 7.2 or more, 80.8% of babies had an Apgar score of seven or more.
The analysis the authors used compared the correlation between the actual fetal blood pH and the actual umbilical cord pH (rather than just whether it was “normal” or not) and found there was a correlation between the two, but it was weak. This was true for all the comparisons they looked at, including base excess levels, and Apgar scores, and the relationship remained weak even when they corrected for confounding variables like mode of birth. The predictive value improved when they use a cut off pH of under 7.05, but there were only 14 babies (3.4% of the sample) with this low level.
The authors concluded by saying that “Fetal blood sampling remains a valuable aid for ruling out low oxygen levels in the presence of an abnormal CTG” – but I’m not really seeing that in the data myself. I do agree with their final statement in their paper:
Critically, the true value of fetal scalp blood sampling in terms of reducing unnecessary medical intervention, recognising fetal hypoxia, and this preventing adverse neonatal outcomes, remains unclear.
Here’s what we need to be thinking about…
The data in this study comes from clinical practice. Clinicians did the fetal blood sampling because they intended to use the results of the test to make recommendations about what to do next. No one was blinded to the results of the fetal blood sample. As a consequence, any relationship between the fetal blood pH results and the outcomes being measured could be due to those subsequent management decisions, rather than whatever led to the pH test results being what they were. Any one of a number of interventions (like ceasing an oxytocin infusion, coached pushing, or the administration of general anaesthesia for urgent caesarean section) might have had an impact on the outcomes that were measured.
The timing of cord severance was not discussed in the paper, but was likely to be early given that all the babies included had cord blood gas analysis and cord clamping is often done to facilitate this. Small differences in the timing of when the cord is clamped can alter pH levels (Olufssen, 2023). Cord pH levels are lower when the circulation remains uninterrupted.
On the face of it, this type of research seems logical, but that is only true because of a misunderstanding of the complexity of the physiological processes involved in fetal heart rate control and responses to falling levels of oxygen. If the assumptions that underpin your research are inaccurate, then what you think you are testing is probably not what you are actually testing after all. While I applaud the research team for their desire to contribute better evidence for clinical practice, what we really need is research based in a deep understanding of fetal physiology that is designed to assess whether the intervention of fetal blood sampling improves clinically significant outcomes for women and their babies.
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References
Godhard, E., Michelotti, F., Hayes, D. J. L., & Heazell, A. E. P. (2025, Mar). Evaluating the diagnostic accuracy of fetal scalp blood sampling: A retrospective cohort study. European Journal of Obstetrics Gynecology & Reproductive Biology, 306, 29-34. https://doi.org/10.1016/j.ejogrb.2024.12.039
Olofsson, P. (2023, May). Umbilical cord pH, blood gases, and lactate at birth: normal values, interpretation, and clinical utility. American Journal of Obstetrics and Gynecology, 228(5S), S1222-S1240. https://doi.org/10.1016/j.ajog.2022.07.001
Categories: CTG, New research
Tags: Apgar, cord blood, fetal blood sampling, pH
Birth Small Talk 
Hi Kirsten OK so what can we do, if all your followers did something, Maybe we could effect an outcome. How do we influence the researchers or the companies to initiate the research? Which companies are interested in actually researching an outcome – if there is no financial gain from what they expect the outcome to be. I’d welcome your thoughts. love Chris
Chris Warren Midwife York Storks Midwifery Support 01423360460
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There are very strong social forces that created and maintain the status quo. I have folks in the Fetal Monitoring Academy beginning to develop skills and confidence with policy change and we’ll start there. There’s no easy fixes.
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