Antenatal CTG monitoring: Does it work?

As I have been refreshing older blog posts, I realised I never did get around to writing about antenatal CTG use. Today’s post solves that gap!

What is antenatal CTG monitoring and when is it used?

Mostly I write about the use of CTG monitoring during labour. But this is not the only time CTGs are used in maternity care. Maternity professionals also use CTGs during pregnancy in an attempt to assess the health of the fetus. This is sometimes called a non-stress test or NST. Some examples of the reasons antenatal CTG monitoring is currently used include:

When a woman has experienced a reduction in fetal movements
When a woman has pre-eclampsia, diabetes, or another health issue that can impact on fetal health
After an episode of vaginal bleeding (antepartum haemorrhage)
After trauma, like a car accident, or
After a procedure, like an external cephalic version aiming to turn a breech presentation to a head first one

How would you prove antenatal CTGs work?

Proving antenatal CTG monitoring works (or doesn’t) is trickier than it appears. First, you have to decide what you are comparing it with. There is more than one possibility here. Antenatal CTG use could be compared with doing no fetal monitoring, of any kind. Or with a brief period of intermittent auscultation, or with repeated episodes of intermittent auscultation (like listening for one minute, every 15 minutes, over a two hour period). It could also be compared with ultrasound assessment of fetal health (measuring fetal size, amniotic fluid volume, and blood flow through important blood vessels).

Do you pick just one of these, or do you try to design a study where all are tested? Care also needs to be taken to control for other variables that might impact on outcomes. If all women randomised to CTG monitoring are offered an ultrasound scan but this isn’t offered to women in the control group, outcomes might be due to the ultrasounds, not to the CTG monitoring.

The next question is, works for what? What outcomes are maternity professionals trying to achieve or avoid by antenatal CTG use? These are similar to the ones for CTG monitoring in labour (neonatal death, cerebral palsy, caesarean section) but also include things like stillbirth before labour, induction of labour, and gestational age at birth. Ideally, you would ask a panel of women with recent pregnancy experience to provide input, so the outcomes you are measuring are relevant to women who will be making decisions about whether to have an antenatal CTG.

You would also need to decide if you were only doing either the intervention or control just once, or repeatedly over time. Any form of fetal monitoring only tells you how the fetus is right now, so if the risk is ongoing, then it is usually to repeat the assessment periodically. We currently have no idea about the ideal interval between assessments, so this would be a guess.

Add finally, good research would specify the CTG patterns that were considered normal, or not normal, and what to do about the not normal ones in advance. We are yet to fully understand how best to interpret heart rate patterns in labour, at term, with even less research having been done to help make sense of preterm, antenatal CTG traces.

What research has been done?

There is a Cochrane review, summarising findings from randomised controlled trials about antenatal CTG monitoring (Grivell, et al., 2015). They found four trials, all published between 1982 and 1985, all done in groups of women considered to be at increased risk for complications. Three were done in the UK and the other in Australia. The sample sizes were small, ranging from 300 to 539 women.

Three of the trials (Brown, et al., 1982; Flynn, et al., 1982; Kidd, et al., 1985) did CTGs on all women in the trial, but half of the women had the CTG trace hidden until after birth. (I simply cannot imagine how the person performing the CTG, who presumably knew something about CTG interpretation, might see an abnormal pattern and shove the trace in an envelope only to be revealed after the baby was born.) The other trial doesn’t clearly describe whether intermittent auscultation was used for women who were allocated to the control group, but did say tests for human placental lactogen, oestriol, and pregnancy specific glycoprotein (long since abandoned from practice) were commonly used in both groups, along with basic ultrasound (Doppler assessment of blood flow was not available then), and kick charts for fetal movement monitoring (Lumley, et al., 1983).

Women in the trials were typically those who had been admitted to hospital, and were at least 26 weeks pregnant. Women had conditions like pre-eclampsia, fetal growth restriction, antepartum haemorrhage, and diabetes. One trial included women around or just after their due date who were managed as outpatients (Flynn, et al., 1982). CTGs were done at different intervals in different studies: some weekly, some twice weekly, and some daily.

Two other trials compared antenatal CTGs where computer interpretation was used with antenatal CTGs where computer interpretation was not used. I’ll cover those another time.

What did they find?

Perinatal mortality rates were 23 per 1000 births in the CTG group and 11 per 1000 births in the no CTG group. The risk ratio here was 2.05 and it just fails to reach statistical significance (95% confidence intervals 0.95 – 4.42). Looking only at potentially preventable deaths (by removing babies with abnormalities not compatible with life), the findings are similar: 17 deaths in the CTG group and 6 in the no-CTG group per 1000 births (risk ratio 2.46, 95% confidence intervals 0.96 – 6.30). Only one trial reported on Apgar scores (no significant difference), one on neonatal seizures (no significant difference), and two on admission to the nursery (no significant difference). Gestational age at birth was not different in the two groups.

Looking at outcomes for women, there were no differences in the use of induction of labour or caesarean section.

What does this mean?

The research is old and the number of women recruited has been too few to show differences in outcome. There was a strong trend towards more deaths occurring when CTG monitoring was used, with a 246% increase in potentially preventable deaths. Yes you read that right. More babies died when CTG monitoring was used.

You would think these results would mean people involved in the research would do a huge WHOA! and drop antenatal CTG monitoring. Ah no… Yet again, we see the absolute and unshakeable faith people have in CTG technology – even when faced with evidence it is actually harmful from their own research. Lumley et al., (1983) tell us that:

Since the trial ended the use of antenatal [CTG] monitoring within the hospital has increased 16-fold.
p. 1025

Sigh.

I find it difficult to find new ways to say how current practice in relation to all things fetal monitoring is nonsensical. Here we have evidence that antenatal CTG use quite probably increases the mortality rate, yet researchers have not thought to revisit these findings (we’ve had almost 40 years – there was time to do it!), clinical guidelines continue to recommend antenatal CTG monitoring, maternity professionals are not taught the evidence, and women are not given accurate information to support their decisions. It is well past time to do better.

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References

Brown, V. A., Sawers, R. S., Parsons, R. J., Duncan, S. L. B., & Cooke, I. D. (1982, Sep). The value of antenatal cardiotocography in the management of high-risk pregnancy: a randomized controlled trial. BJOG: An International Journal of Obstetrics and Gynaecology, 89(9), 716-722. https://doi.org/10.1111/j.1471-0528.1982.tb05097.x

Flynn, A. M., Kelly, J., Mansfield, H., Needham, P., O’Conor, M., & Viegas, O. (1982, Jun). A randomized controlled trial of non-stress antepartum cardiotocography. BJOG: An International Journal of Obstetrics & Gynaecology, 89(6), 427-433. https://doi.org/10.1111/j.1471-0528.1982.tb03631.x

Grivell, R. M., Alfirevic, Z., Gyte, G. M. L., & Devane, D. (2015, Jun 26). Antenatal cardiotocography for fetal assessment. Cochrane Database of Systematic Reviews, 9(9), CD007863. https://doi.org/10.1002/14651858.CD007863.pub4

Kidd, L. C., Patel, N. B., & Smith, R. (1985, Nov). Non-stress antenatal cardiotocography: A prospective randomized clinical trial. BJOG: An International Journal of Obstetrics & Gynaecology, 92(11), 1156-1159. https://doi.org/10.1111/j.1471-0528.1985.tb03029.x

Lumley, J., Lester, A., Anderson, I., Renou, P., & Wood, C. (1983, Nov). A randomized trial of weekly cardiotocography in high-risk obstetric patients. BJOG: An International Journal of Obstetrics & Gynaecology, 90(11), 1018-1026. https://doi.org/10.1111/j.1471-0528.1983.tb06439.x