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.
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’. 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.
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.
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…