I have PWS on the inside and ouside of my lip and upper gum.
My PWS did not stretch at all, it just grew with my face as I got older. but it did not stop growing. Some doctors say that hormons make the birthmark grow. In my case, it diffently has grown. I have had plastic surgery to remove some of the extra tissue in my lip. Not everybodies will grow since everyperson is different.
I hope this helps.