Purple line & dilation

Alright mommas.. this is my second baby but I have NEVER heard of this before. Last night I was looking up things about labor because I’ve been experiencing TERRIBLE “lightning crotch” so bad it makes me stop what I’m doing and I have to hold back from crying out in pain. Anyway, I came across this thing that said as women begin to dilate there is a purple line that will appear. It begins at the anus and progresses up the women’s “crack”, the further up it is tells how dilated you are! Have any of you experienced this or heard of it?! If so how accurate was it? I noticed that I have one and if it is a thing and I’m correct I should be about 4-5cm dilated!! I have an appointment tomorrow but I’m just curious if anyone else has experienced or heard of this before?!!!