Cervical dilation is (now mathematically shown to be) unpredictable

The unpredictable cervix

The idea that cervical dilation is unpredictable will not come as a shock to most people who read my books or blog. It’s also a key theme in my book, Midwifery Wisdom.

And many people don’t need research to confirm their experiences.

But I think it’s important to share the findings of studies which add weight to midwifery knowledge and/or progress our thinking about aspects of labour and birth, so it’s in that vein that I offer this blog post.

I’m not going to repeat everything I’ve written in Midwifery Wisdom. There is an entire section in it called ‘At Your Own Pace’, which looks at issues relating to labour progress.

This blog post simply explains why, as some would say, ‘the maths isn’t mathing…’

The medical approach to prediction and birth

One of the fundamental ideas held by some of the people who take a medical approach to childbirth is that it should be possible to research labour and birth, find the ‘rules’ that govern this, and then use those rules to be able to predict what will happen for individual women, babies, and/or labours.

This idea is great (at least for some people) in theory, but it is just a theory.

Just because people believe that something is true, or should be true, doesn’t make it true.

And, while lots of research has been carried out to try to measure labour and map it onto a model that can then be applied to all women/births, we haven’t succeeded in creating a model that can predict how birth will unfold for the individual.

Dilation is unpredictable

In fact, some of this research has helped undermine the very ideas that we can predict how a woman’s cervix will dilate during labour and birth, and that mapping women’s labours onto some sort of chart can help us determine who is having problems while not over-intervening in the labours of women and babies who are doing fine.

The first study that I saw on this topic was published in 2015. A team of researchers in Italy confirmed, using some genuinely clever maths, that in spontaneous, naturally progressing labour, the rate of cervical dilatation is largely unpredictable.

This finding was genuinely interesting and paradigm-challenging then, and it still is now.

Except that the paradigm and the obstetric belief in the possibility of birth prediction has continued, despite the fact that we’ve seen several other studies whose results show the same thing.

Some of the authors of these studies have specifically stated that cervical dilation is not a good measurement for predicting the outcome of labour and birth.

More evidence, the same results…

For example, a large 2018 study in the BJOG, involving almost 10,000 women across thirteen hospitals in Nigeria and Uganda, looked specifically at whether cervical dilatation over time could predict which women would go on to experience severe adverse outcomes.

The researchers concluded that “labour is an extremely variable phenomenon, and that dilatation over time is a poor predictor of adverse outcomes” (Sousa et al 2018).

They suggested the one centimetre an hour myth needs re-evaluating. (As above, please see Midwifery Wisdom for more studies on this topic; I’m just giving a taster in this blog post.)

More recently, a systematic review by He et al (2023) in the American Journal of Obstetrics & Gynecology tried to draw together the evidence from all the labour curve studies published since Friedman’s original work in the 1950s.

This proved somewhat difficult.

What’s the problem?

One aspect of the problem for researchers like He et al (2023) is that, even among the studies that specifically looked at acceleration and deceleration phases of labour, researchers can’t agree on when these phases even begin.

Some placed the acceleration phase between 2 and 4cm, others between 4 and 5cm, others 5 to 6cm.

The researchers concluded that, “…there is an emerging consensus that the definition of abnormal labor may not be related to an idealized or average labor curve.” (He et al 2023).

This is, again, acknowledgement that it’s the paradigm itself that is flawed.

It’s not the maths

A 2024 scoping review pulled in over a hundred studies on labour curves and arrived at much the same place: we cannot find a single reliable model which maps how dilation progresses.

While some researchers argue that we just need better maths, this isn’t the logical conclusion from the data itself.

The data strongly suggest that there IS no one model.

The data support the idea that variation itself is normal.

An unchanging picture

So the picture hasn’t changed; it’s just got more evidence behind it.

Women who are left to their own devices will labour at their own pace, and variation is normal.

Some women labour faster, some slower.

Sometimes this varies in response to what’s going on around them.

Sometimes normal, healthy labour will pause.

None of this happens as predictably as some people would like, but that doesn’t make it wrong.

It actually provides more evidence of why we need a midwifery-focused approach.

The value of research

It can be good to have studies to back up this kind of knowledge, and not just because we live in a world that seems to demand scientific proof of everything, regardless of whether that’s the best way of investigating everything we want to know.

However, I once noted that watching scientists try to measure and predict the rate of cervical dilation in labour is a bit like watching a cat chase the beam of light from a torch someone’s shining on the wall. Utterly pointless fun for a bit, but not something you’d want to spend all day doing.

But the behaviour of the researchers can tall us a huge amount about their approach.

How open-minded are researchers?

Good scientists are open to the possibility that their theories are wrong, or that they only partially explain what theya re trying to understand. When their findings disagree with the existing theory, they will acknowledge what their results are showing, and draw conclusions — like those that I’ve shared in this post — which acknowledge that the existing theory may be incorrect.

However, some people are so attached to their beliefs that they either can’t or won’t acknowledge what the results are really showing. Rather than being intellectually honest with themselves or others, they either try to squeeze their results into their existing belief system, or continue to carry out research, perhaps subconsciously designing it so the results will support their beliefs. Some find it so painful that they will manipulate the data to suit their theory or existing beliefs.

When this happens, it’s women, babies, and families who suffer. And not just because the continued focus on the cervix affects the experiences of those giving birth. More women are asked to participate in studies, which means they asked to undergo even more vaginal examinations than usual so that enough data can be gathered.

But what can we do with this knowledge?

I differ slightly from the researchers on what we do with the knowledge that cervical dilation is unpredictable.

The Italian team in the first study I mentioned above suggested that future research should focus on diagnosing the causes behind apparently erratic cervical changes.

I’m still not convinced that diagnosis is a necessary feature of a normal, physiological process.

And, again, I think the body of research on this topic clearly indicates that there are no simple, linear relationships waiting to be uncovered here, however good the maths gets.

So how about we redirect some of that research funding into teaching more people to watch and respect? Maybe even to be patient?

From prediction to knitting, and beyond

One of the other things I discuss in Midwifery Wisdom is the value of knitting. I was a student midwife in the days where we had adequate enough staffing on British maternity units that midwives could bring their knitting onto night shifts, and knit in the quiet periods when women and babies were sleeping.

Some midwives would also knit while women were in labour. Again, this was when we had adequate staffing, and we could sit quietly in the corner of the room and knit. It was a really good way of conveying to women and families that all was well; that we weren’t worried, and that we were patiently waiting for things to progress at their own pace.

My focus on discussing the value of knitting isn’t because I think it’s possible to return to this trend in modern maternity care. I’m well aware that it isn’t. I mention it as just one reminder of how much things have changed, and of what we’ve lost over the past few decades.

It’s fascinating to me that knowledge from scientific and mathematically-based research supports this approach, rather than the constant measuring, meddling and manipulation of labour and birth that we see today.

As the evidence keeps confirming, cervical dilation — like many aspects of birth — is unpredictable.

And if dilation can’t tell us what’s going to happen, then the most useful thing we can offer a woman in labour might not be a focus on measurement, but our patience.

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