Purple Line Dilation - What is the Purple Line in Labor?
Purple Line Dilation “Club“
More parents are opting OUT of uncomfortable routine vaginal examinations in labor and prenatally (without a medical indication) and are opting IN to the purple line dilation club as a way of assessing labor progress. Research suggests that around 50% of vaginal examinations in labor are inaccurate - and the further along in labor you get the less accurate they become. A brand new 2021 study suggests it is about 85% accurate for dilation and how far down into the pelvis baby is. Yes dilation is important - everyone is obsessed with that number but the station of your baby (how high/low in the pelvis) is crucial information too. You can be fully dilated and baby can still be high in the pelvis (not close to the ‘exit’). Ideally being at +2 ‘station’ is when you’ll start to feel that strong urge to push. If you have an epidural allowing your body to nudge baby down takes a LOT of the ‘work’ out of pushing.
So why not consider alternative ways to assess progress that reduce infection, discomfort and keeps you off the bed and may help you to stay home longer. Combined with other signs it’s a simple way to learn how to tell if you’re dilated yourself - before you go to hospital.
So let’s talk about the Purple Line!
(I’m not suggesting you actually get out the measuring tape - the feeling of being observed, poked and prodded even at home can disrupt labor hormones - but if you have a birth partner they can keep an eye on it).
Hobbs Line - Also known as the Red/Purple Line in Pregnancy
Hobbs line (named after the researcher who published the first paper on this topic) is also known as the purple line has been getting more well deserved attention recently as an alternative, less invasive way to assess labor progress. It may even be more accurate than a vaginal examination given that some studies have shown that the overall accuracy for cervical dilation is only between 48% to 56% - and the more dilated you are the less accurate the VE (vaginal examination).
The purple line acts like a mercury thermometer - sometimes it can seem more like a red line between the upper buttocks and the higher it goes the more progress you’re likely to making. As the length of the line increases so does dilation - good to know eh? red line between upper buttocks
What Causes the Purple Line? Is Pressure in the Bum a Sign of Labour?
As your baby descends deeper into the pelvis, a red/purplish or perhaps brown (depending on skin color) line creeps up from the anus to the top of the natal cleft in between the bottom cheeks. It’s thought to be caused by blood congestion in this area due to the pressure of baby’s head. A 2018 study suggests it’s seen more often if your waters released in early labor. Several studies suggest as many as 86% of parents will experience this phenomena.
The main causes for appearance of purple lines are unknown, but many scientists have been proposed that the vaso-congestion at the basis of the sacrum can cause the increase of the appearance of purple line. It lightens with pressure and slowly re-colours once the pressure was decreased. These congestions appeared due to the growing of intra- pelvic pressures as the fetal head descends, that considered appositive correlation between the decent of the foetal head and increase the presence of purple-lines. In addition, ordinary change in the shapes of the pelvis which the rhombus of Michaelis of the bones moves back throughout advanced birth, pushing out the ilea wings and growing the pelvic diameters attach another explanation to the presence of purple line [Bonilla, et al., 2016].
Rhombus of Michaelis Bulge
(Quick aside) Take a look at this incredible image from North Dallas Doulas showing how the pelvis opens to facilitate baby moving through the pelvis. That kite shaped bulge is the Rhombus of Michaelis, which includes three lower vertebrae that move outwards during the second stage of labor (pushing). There’s no mention of the purple line in this image and it’s hard to tell if it’s there - I’d love to know!
Rhombus of Michaelis Image
The purple line was first mentioned in The Practicing Midwife journal back in 1988 and has been steadily receiving more and more attention in the birth world - it’s something your doula might be looking out for at home with you.
“My midwife was explaining this to a student as I was in transition. The student asked if she was going to perform an exam but she said it was completely unnecessary as she could see I was fully dilated by the line. Amazing!” - Donna
Purple Line Dilation Image
Purple line dilation image - source unknown
Do yourself a favor and take a butt selfie today (before labor) just in case you already have some kind of pigmentation there already - then you can compare lines on the big day if you are happy to stay home longer. Take it in the 3rd trimester when you’re getting ready for your perineal massage.
Always look for other signs that labor is progressing and don’t rely on the purple line alone (mucousy birth show, strong surges or your waters have released) and growing pressure in the bum as a sign of a progressing labor.
Pregnancy Purple Line Dilation Pictures
Image credit Lamaze International
Why haven’t you heard of it before? It might have something to do with the fact some parents are not encouraged to mobilise in labor and a significant number give birth on their back - so it’s not something most staff are looking for - or even know to look for.
“I have a picture of my purple line! Having my last baby the midwife said to us " there you go you’re at 10 cms" we asked how'd she know as I asked for no VE's and she said because of the line! “- Jennifer
Pregnancy Purple Line Dilation Image credit - First Glance Photography
“We discussed this while I was in labor on Saturday. Apparently I did have the line and my birth photographer took some pics. Can't wait to see them!” - Aoife
Natal Cleft Picture - Image source unknown
Birth doulas and midwives are usually more au fait with the more subtle ways of observing labor progress. Read more about cervical checks and how you can do your own at home before going to hospital or your birth center.
Talk to your careprovider and their experience of assessing labor progress with the purple line.
I look forward to the day when we see something like this sentence in written birth preferences.
“Due to the inaccuracy of vaginal examinations, discomfort caused and risk of infection I prefer to have my labor assessed visually with a purple line check.” Maybe add it to these questions when chatting to your provider.
Purple Line Dilation Chart
Purple Line Dilation Chart - Farrag et al (2021)
Would you prefer this way of assessing your labor progress (along with other signs such as more birth show, internal focus, increasing feeling of pressure and other visual (and audible) cues?
Tracy
Resources
Hobbs 1998. Assessing cervical dilatation without Vaginal Exams. Watching the purple line. The Practising Midwife 1(11):34-5.
https://bmcpregnancychildbirth.biomedcentral.com/articles/10.1186/1471-2393-10-54
https://repositorio.animaeducacao.com.br/bitstream/ANIMA/2655/1/2018_Nunes_Traebert_purple_line.pdf
Huhn KA, Brost BC. Accuracy of simulated cervical dilation and effacement measurements among practitioners. Am J Obstet Gynecol. 2004;191(5):1797–9.
Letic M. Inaccuracy in cervical dilatation assessment and the progress of labour monitoring. Med Hypotheses. 2003;60(2):199–201.
