In this blog post, Dr Sara Wickham gathers together evidence for the purple line in labour.
The ‘purple line’ is a line of temporary skin discoloration that can be seen in the anal cleft of some women as they progress in labour.
In early labour, the line is short, and it gets longer as labour progresses.
This means that some midwives use it as a sign of labour progress. It’s one of a number of non-invasive approaches that we can use to assess how a woman/baby is progressing in labour.
The purple line
I have been aware of the purple line being watched for, used and discussed by midwives for my whole career. I suspect that it has been known about and discussed for much longer than that.
In more recent times, people (including myself) began to write about and research it. And it’s those studies and papers that I’m going to discuss in this blog post.
A few things to know
But there are a few important things you should know about the purple line before I get into the evidence.
First, it is seen a bit more easily in women with lighter skin. More on this below.
The so-called purple line is sometimes more red than purple. It can also be blue, brown, black, whitish, or pink, and I’m sure there will be more colours to come. So don’t let its name fool you.
Why do I still call it purple, given that it can appear as many colours?
Well, mostly because I work with academic and research texts, and that’s how it is currently described in those. It’s what people are searching for, and using any of those colours in the search box instead would bring up a LOT of articles which don’t relate to this phenomenon at all.
Equally, if I used other terms, my work wouldn’t come up when others searched on the term that has historically need used. This is just one reason why it’s really important to think about the wider context of knowledge, language and terminology.
Positions matter
And it is easiest to observe if a woman is leaning forward or on all fours.
The purple line can also be seen if the woman is lying on her side.
So it is not quite such a useful tool when women choose positions other than this.
Many women far prefer having a midwife look for the purple line than doing a vaginal examination.
Understandably, though, some don’t want to have practitioners peering at their bottom either.
And there are wider questions about to what extent we really need to assess progress in labour, but that’s another story.
Let’s start with the studies.
Byrne & Edwards (1990)
The purple line was first discussed in the literature (to my knowledge, but please see below) in a letter written to The Lancet by Byrne and Edmonds (1990).
They described the purple line as, a ‘clinical sign which will indicate the progress of the first stage of labour without vaginal examination’ (122).
Interestingly, they attributed a Sister H Lake as having first observed the purple line.
I think we can safely assume that Sister Lake was a midwife. Sadly, I can’t confirm that. I have previously made quite a lot of effort to track her down, but without success.
Byrne and Edmonds’ (1990) letter described how they had investigated the purple line in 48 women.
They saw the red line on 89% of occasions, but noted the existence of some women in whom the purple line wasn’t seen at all.
Later research would confirm this and offer some interesting findings, but we’ll come back to that…
Physiological origins
Byrne and Edmonds (1990) shared their ideas about the possible physiological origins of this sign:
‘The line may arise because of vasocongestion at the base of the sacrum. It blanches when firm pressure is applied and slowly recolours when pressure is withdrawn. This congestion possibly occurs because of increasing intrapelvic pressure as the fetal head descends, which would account for the correlation between station of the fetal head and red line length …
We postulate that increase in intrapelvic pressure causes congestion in the basivertebral and intervertebral veins around the sacrum, which, in conjunction with the lack of subcutaneous tissue over the sacrum, results in this line of red purple discoloration.” (122).
Hobbs (1998)
I’m not sure how many midwives were aware of Byrne and Edmonds’ (1990) article when it was first published, but many became aware after Lesley Hobbs, a British independent midwife, wrote about this in 1998.
Lesley explained Byrne and Edmonds’ theory and discussed her own observations of the purple line in women, noting among other things that ‘there is a longer gap between 4 and 7cm dilatation than there is both before and after’ (Hobbs 1998: 34).
Although little happened on this in the literature for the next few years, this didn’t mean the purple line was forgotten.
I saw evidence both in my day-to-day conversations and in the data I collected for various research studies that midwives (and a couple of obstetricians) were talking about the purple line and using this knowledge in practice, and I spoke about it at several conferences.
The experience of midwifery colleagues I met in countries such as Trinidad and Jamaica confirmed that this sign is sometimes less or differently visible in women with darker skin.
Shepherd et al (2010)
In 2010, Shepherd et al published the results of the first formal research study to look at evidence for the purple line.
They observed the progress of 144 women in labour. These researchers were based in Scotland, so theirs was a predominantly white population. They saw evidence of the purple line in 76% of the women.
These researchers hadn’t set out to find out more about the physiology of the purple line, but their results showed a medium positive correlation between the length of the purple line, the dilation of the woman’s cervix and the station of the baby’s head.
One of their findings that I find the most fascinating is that the line was far more likely to appear when a woman was in spontaneous labour. We have no idea why this is the case. However, as I discussed in In Your Own Time, there are many advantages to going into spontaneous labour, and it may be that the purple line reflects the body being physiologically ready for birth.
Narchi et al (2011)
The following year, Brazilian researchers published the results of a descriptive, prospective, observational study in 100 women in labor at a normal delivery clinic in the State of São Paulo.
They saw evidence of the purple line in 56 of the 100 women who were observed during the active phase of labour.
It should be noted that more than half of the participants in this study described themselves as Black or Brown. That’s relevant because, as above, midwives at the time were reporting seeing it less in women with darker skin. The line was present in 27 of the 40 white women (67.5%) and 29 of the 60 non-white women (48.3%) in the Brazilian study. The authors note that, “the difference between the racial subgroups was not statistically significant,” but this may well be because the study was small.
The researchers also report a statistically significant correlation between the measurements for dilation of the cervix and those for the line.
In 22% of the women, the purple line also became discoloured during labour. (I take ‘discoloured’ to mean changed colour, and this is something that has been noted by others. However I do not speak/read Portuguese and am relying on a translation of the article.) However, there was no association between the this discoloration and the extent to which the head of the fetus had descended in the cervix.
For 81% of the women, evaluation of the purple line in labour caused no discomfort whatsoever. When interviewed after childbirth, the other 19% said they felt ashamed, embarrassed or bothered. This was principally because they were asked to adopt the all fours position for the examination.
By contrast, however, 91% of the women expressed negative feelings about vaginal examination. They reported pain (7%), discomfort (40%), feelings of embarrassment (4%), disrespect (4%), fragility (1%) and fear (1%). Only 9% of the women stated that a vaginal examination had not caused them any discomfort.
Narchi et al (2011) concluded that measurement of the purple line could be used to reduce the number of vaginal examinations, especially when the only aim of the examination is to measure dilation of the cervix.
Kordi et al 2014
The next study that I’m aware of was carried out in Iran.
“In this cross-sectional study, 350 women with a single pregnancy in vertex presentation and gestational age of 38-42 weeks without any medical disorder, admitted to government hospitals of Mashhad, were selected using convenience sampling. Vaginal examination and observation of the line each hour in the active phase of labor were measured.”
“The purple line appeared in 75.3% of women during the active phase of labor. Appearance of the purple line in the prediction of labor progress had 90.2% sensitivity, 45.3% specificity, 88.1% positive predictive value, 51.0% negative predictive value in the first stage of labor and had 87.6% sensitivity, 52.4% specificity, 96.5% positive predictive value, 22.0% negative predictive value in the second stage of labor and has 68.57% sensitivity, 42.66% specificity, 85.32% positive predictive value, and 43.85% negative predictive value for the total labor.”
“According to the appearance of the purple line in most of the cases and its high sensitivity and specificity, we can use it as a non-invasive complementary method for clinical assessment of labor progress.” (Kordi et al 2014).
The Kordi et al (2014) study was also published in The Journal of Obstetrics and Gynaecology by Irani et al (2018). Irani was the second author on the Kordi et al (2014) paper.
This time, the researchers noted their concern that, “Vaginal examinations can be unpleasant, intrusive, embarrassing and so painful for women.”
You’ll find more on this, and many other topics, in my new book, Midwifery Wisdom.
In it, I explore the knowledge that experienced midwives develop over years of supporting women through pregnancy, birth, and the postnatal period. How midwives use their senses to assess labour progress, why the maths behind obstetric boundaries needs rethinking, the value of postnatal tea and toast, and when to put your gloves on. I cover topics from risk to cats in a book written to help people understand midwifery, and midwives better understand the value of their own skills and wisdom.
Wider conversations
I want to pause to note that the purple line was being discussed in other ways as well at this time.
I and a number of midwifery colleagues were discussing this at conferences, and raising questions about non invasive methods of assessing the progress of labour.
We were also raising questions about whether the progress of labour needed to be assessed in the way that the technocratic model argued that it did.
A number of papers questioning the practice of vaginal examination were also being published.
Nunes et al 2018
Nunes et al (2018) published their findings in a letter to the International Journal of Gynecology and Obstetrics.
In their study, the purple line was seen in 189 of the 220 women, which is 85.9%.
They noted the presence of the line in both spontaneous and induced labours, and also speculated that it might also be associated with premature rupture of membranes.
Nunes et al (2018) also found that white ethnicity was associated with a 12% higher chance of identifying a purple line.
They concluded that the purple line, “…had high incidence but was not suitable for diagnosing cervical dilatation and fetal head station owing to low accuracy.” (Nunes et al 2018).
Farrag et al (2021)
The next published study was carried out in Egypt and included 120 labouring women.
Farrag et al (2021) found that, “the purple line appears among about four fifth of the total sample. A medium positive association was found among the purple line length, dilatations of the cervix and fetal stations. The purple line appearance in the expectation of labour progress had 87.91% sensitivity, 39.53% specificity and 85.25% accuracy.”
Like Shepherd et al (2010), Farrag et al (2021) found that the purple line was more likely to be seen in women experiencing spontaneous labour than in those whose labours had been induced.
However, they found no significant association with parity, age, BMI or the baby’s birth weight. We must again remember that these studies are small. It’s also not clear in many of them how the samples were selected and whether this could have influenced the findings.
Farrag et al (2021) concluded that, “The purple line is useful in assisting the health care providers in assessing and determining labour progress and lessen the number of vaginal examinations, particularly at onsideration of females who refuse or feel that the examining intrusive.”
Papoutsis et al‘s (2023) systematic review
More recently, a systematic review by Papoutsis et al (2023) brought together these studies on the purple line. The authors included all of the studies that I have shared on this page.
Here are their findings:
“There were six eligible studies included in the systematic review that reported on 982 women in total with the purple line appearing in 760 (77.3%) of cases.
We found a moderate positive pooled correlation between the purple line length with cervical dilatation (r = +0.64; 95%CI: 0.41–0.87) and fetal head descent (r = +0.50; 95%CI: 0.32–0.68).
For women either in spontaneous or induced labour, the pooled mean length of the purple line was more than 9.4 cm when the cervical dilatation was 9–10 cm, whereas it was more than 7.3 cm when the cervical dilatation was 3–4 cm.” (Papoutsis et al 2023).
Yildirim et al (2025)
Researchers observed 304 women experiencing term, active labour in a hospital in Turkey.
With the women’s consent, they measured the purple line hourly, with a disposable tape measure.
Their results confirm the findings of several other studies.
“A purple line was seen in 85.9% of pregnant women at any stage of labour, with a specificity of 86% and sensitivity of 65%.
During the active phase of labor, the measurements of the purple line length in the sacral region increased proportionally with cervical dilatation.
The sensitivity was 56%, and the specificity was 65% when the fetal head descent reached the − 2 level.” (Yildirim et al 2025).
Sensitivity and Specificity
These researchers considered the sensitivity and specificity of the purple line, so let’s take a moment to look at what that means.
Sensitivity and specificity are a way of describing how well the measurements that the midwives took of the purple line matched (or correlated with) the assessment of cervical dilation that they found on vaginal examination. They also looked at how well the purple line measurement correlated with the descent of the baby’s head through their mums’ pelvis.
In simple terms, sensitivity measures how good a test (in this case the purple line measurement) is at correctly identifying women whose cervixes are about as dilated as the measurement suggests when a vaginal examination is used to measure dilation instead.
(Yes, there is a lot of evidence showing that vaginal examination is quite subjective, and that different people often get different measurements. We also know that the cervix doesn’t behave in the linear way that the obstetric mindset would like us to think it does. But I’m going to ask you to set those thoughts aside for now, because we don’t have anything better to measure the purple line against.)
Specificity measures how good the purple line measurement is at only identifying women who are at a particular stage of dilation. In other words, we want to make sure that we aren’t incorrectly saying that women’s cervixes are 6cm or 8cm dilated when they’re actually only 2cm dilated.
A highly sensitive test rarely misses confirming the dilation found on vaginal examination, and a highly specific test rarely identifies a woman’s cervix as being at a particular dilation if it is not.
So how good IS the purple line?
The sensitivity and specificity varied a bit, depending on the stage of labour. The researchers found that the purple line was a more accurate reflection of cervical dilation when the woman’s cervix was about 5-6cm dilated than at the very beginning or end of labour. And this study included only caucasian women, so the skin colour issue is again an important caveat. But overall the sensitivity and specificity were reasonable, especially given that, as above, vaginal examination itself is subjective.
The paper is freely available here if you want to look into the detail of that.
This is a useful study, and as the researchers note, “…the purple line in the sacral region is a non-invasive method to assess the course of labor and can be used as an adjunct to vaginal examinations and may reduce the number of vaginal examinations in clinical settings.” (Yildirim et al 2025).
Issues with methods
As I have noted throughout this post, all of these studies are small. For this and other reasons, we can’t be as certain about their findings as we could if they had been designed differently. Papoutsis et al (2023) have discussed the issues with the research methods at length. Both Shepherd et al‘s (2010) study and and Papoutsis et al‘s (2023) discussion should be useful to anyone looking to do more research.
It’s important to note, however, that it is notoriously difficult to carry out large studies on topics like this one. Large studies require significant funding. Funding tends to come from pharmaceutical and technology companies, and from government-affiliated bodies.
Where something is low-tech, less invasive and presents a challenge to the status quo of the obstetric paradigm, it’s next to impossible to get funding to research it.
I write from experience.
This is why researchers (like myself) who want to explore the non-technocratic possibilities often seek support for their work in creative ways, and/or carry out small studies, which are far less expensive to run.
In conclusion…
So there is definitely room for more research, but we have learned a few things from these small studies.
The purple line can be a useful means of estimating the progress of labour in women whose bodies show this sign.
It’s not infallible, exact, or perfect, but then neither is vaginal examination. As vaginal examination is painful, uncomfortable and embarrassing for many women, we would do well to keep exploring other means of estimating labour progress.
It would be fascinating to know why the purple line seems to appear more often when women go into labour themselves.
The purple line can be seen in darker skin more often than we once thought, often appearing as a black line, but not as often as it can be seen on white skin. We clearly need to spend more time learning how to assess how this and other clinical signs may present differently in people with different skin tones.
I’m excited to see what we learn about this and other less invasive means of assessing labour progress in years to come.
What else can we learn?
Finally, I said above that Byrne and Edmond’s (1990) letter was to my knowledge the first time that this sign was discussed in the literature.
Having seen the purple line many times myself, though, I find it hard to believe that Sister Lake was the first birth attendant to notice it.
I often wonder whether, one day, someone doing some historical midwifery research will come across an even earlier reference to this in old texts.
Or perhaps in other eras it was so well known and accepted that there would have been no need to even mention it.
An important note
One last note, which I have added after seeing some social media posts on this topic.
If you see someone claim that they can tell exactly how far along a woman is by the purple line, that they can always see it, or that it’s a better means of assessing progress than vaginal examination, that’s a time to worry about their honesty or integrity.
Phenomena like the purple line offer us a rough guide, and do not constitute an exact science.
But the same is true of every other assessment of progress, including when we measure cervical dilation by vaginal examination.
Epistemological humility is a wonderful thing. In fact, I’ve written more about this…
You’ll find more on this, and many other topics, in my new book, Midwifery Wisdom.
In it, I explore the knowledge that experienced midwives develop over years of supporting women through pregnancy, birth, and the postnatal period. How midwives use their senses to assess labour progress, why the maths behind obstetric boundaries needs rethinking, the value of postnatal tea and toast, and when to put your gloves on. I cover topics from risk to cats in a book written to help people understand midwifery, and midwives better understand the value of their own skills and wisdom.
References
Byrne DL & Edmonds DK (1990). Clinical method for evaluating progress in first stage of labour. Lancet 335(8681): 122.
Farrag R, Abd N, Eltohamy E (2021). Accuracy of purple line to monitor labour progress: Longitudinal study. Egypt J Health Care, 12 (1): 30-44.
Hobbs L (1998). Assessing cervical dilatation without VEs: watching the purple line. TPM 9(11): 34-35.
Kordi M, Irani M, Tara F et al (2014). The diagnostic accuracy of purple line in prediction of labor progress in Omolbanin Hospital. Iran. Iran Red Crescent Med J 16: e16183.
Narchi N, Camargo J, Salim N et al (2011). The use of the “purple line” as an auxiliary clinical method for evaluating the active phase of delivery. Rev Bras Saude Matern Infant 11: 313–322.
Nunes RD, Locatelli P, Traebert J (2018). Use of the purple line to diagnose cervical dilatation and fetal head station during labor. Int J Gynaecol Obstet 141(2): 250-251.
Papoutsis D, Antonakou A, Gornall A et al (2023). The purple line and its association with cervical dilatation in labour: A systematic review and meta-analysis. Eur J Obstet Gynecol Reprod Biol 289: 91-99.
Shepherd A, Cheyne H, Kennedy S et al (2010). The purple line as a measure of labour progress: A longitudinal study. BMC Pregnancy and Childbirth 10 (54).
Yildirim AD, Esencan T & Ata B (2025). Alternative approach to monitoring labor: purple line. BMC Pregnancy Childbirth 25, 180.
About the Author: Dr Sara Wickham is an author, speaker, and researcher specialising in pregnancy, birth and maternity care. Her work focuses on evidence-based, woman-centred information and informed decision-making, drawing on more than 30 years of midwifery knowledge and experience. Her most recent book is Midwifery Wisdom.
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