Evidence for the purple line…

small__3975873809Todays answer is to a question that I get asked at least once a month:

Can you help me find references / evidence relating to the purple line? 

And yes, I can.  There aren’t many at all, but there are a couple of important articles and a research study for those wanting to read more and/or write about this sign of labour progress.

Firstly, and especially for anyone who isn’t already aware of this, 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.  Some midwives are good at telling how far along a woman is in her labour by looking at the purple line.

A couple of other important things to know about this are that it is seen far more easily in women with lighter skin, it is sometimes more red than purple, and it is far easier to observe if the woman is leaning forward or on all fours.  Which means that it is not quite so useful when women choose positions other than this and/or if they don’t want to have someone staring at their bottom – which I don’t think is at all unreasonable!!

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), who described it 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 can only assume that Sister Lake was a midwife, and have previously made some efforts to track her down, but without success.

Byrne and Edmonds’ (1990) letter described how they had investigated this 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…

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 It blanches when firm pressure is applied and slowly recolours when pressure is withdrawn.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).

small__11136102954I’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.  She 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 several 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 in countries such as Trinidad and Jamaica confirmed that this sign is less visible (and thus less useful) in women with darker skin.

In 2010, Shepherd et al published the results of their research study into the purple line.  In an article that is freely available, they observed the progress of 144 women in labour and saw evidence of the purple line in 76% of the women. These researchers hadn’t set out to find out more about the actual 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…

There is absolutely room for more research, and I would love to hear from anyone who has anything to add on this subject.  While acknowledging that this sign is only useful in some settings and women, I find it really useful.  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…


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Byrne DL & Edmonds DK (1990). Clinical method for evaluating progress in first stage of labour. Lancet 335(8681): 122

Hobbs L (1998). Assessing cervical dilatation without VEs: watching the purple line. TPM 9(11): 34-35.

Shepherd A, Cheyne H, Kennedy S, McIntosh C, Styles M, Niven C. The purple line as a measure of labour progress: A longitudinal study. BMC Pregnancy and Childbirth 10 (54).


photo credits: Parvin ♣ and antonychammond via photopin cc and cc