Among the many physiological signs of labour progress, the purple line – as it is currently known – has been widely noted by birth professionals. Observed as a darkened streak extending from the anus upward along the natal cleft (the crease between the buttocks), this line is believed to be an indicator of cervical dilation. However, rather than being a separate phenomenon, it is better understood as a direct result of vascular congestion caused by increased pressure and blood flow in the sacral area. For this reason, we should start thinking of it as a congestion line instead.
Why We Need to Rethink the Term ‘Purple Line’
The term purple line has been widely used, but it is both inaccurate and exclusionary. The colour of this line varies significantly depending on an individual’s skin tone and vascular response. While it may appear purplish or reddish on lighter skin, it can be darker brown, grey, or even not clearly visible on deeper skin tones. By calling it a congestion line, we move away from a misleading colour-based description and towards a name that accurately reflects its underlying physiological cause: increased pelvic pressure and blood flow.
What Is the Congestion Line?
The congestion line is a subtle but visible physiological marker that appears as labour progresses. It has been noticed primarily by midwives and doulas as a potential alternative to frequent vaginal examinations for assessing cervical dilation. Some studies and anecdotal evidence suggest that as the cervix dilates, this line extends further up the natal cleft, potentially reaching the top by the time full dilation (10 cm) is achieved.
The Physiology Behind the Congestion Line
The most plausible explanation for the appearance of the congestion line is vascular congestion. As labour intensifies, pressure increases in the pelvis, leading to a build-up of blood in the sacral region. This may cause the superficial veins in the area to become more prominent, resulting in visible darkening. The autonomic nervous system, responsible for regulating blood flow and tissue changes, may also play a role in this phenomenon.
Is the Congestion Line a Reliable Indicator of Dilation?
The reliability of the congestion line as an assessment tool remains debated. Some birth professionals find it to be a consistent indicator of dilation, whereas others report variability in its presence and length. Factors such as skin tone, positioning, and individual differences in blood flow could all influence whether or not the line is visible in a given person.
However, as a non-invasive observational tool, the congestion line offers a compelling alternative to routine vaginal examinations, which can be uncomfortable or even disruptive to the labouring woman. As a secondary sign of dilation rather than a direct measure of cervical change, its presence could provide useful insights for those attending a birth, offering an additional piece of the puzzle when assessing labour progress.
Final Thoughts
The line currently referred to as the purple line is more accurately described as a congestion line, reflecting the physiological changes that cause it rather than a colour-based descriptor that does not apply to all skin tones. As a non-invasive tool, it deserves further study and discussion. For birth professionals, recognising the congestion line’s role in assessing labour progress could provide additional support for those giving birth.
Have you observed the congestion line in your practice? Share your thoughts and experiences in the comments below!