OK a bit of weird one this.
My wife gave birth to DS1 about 9 years ago but when she was pregnant she started to develop a vertical groove running vertically down the centre of her forehead from her hairline to her eyebrow line. It is shallow, fairly wide and distinct. She thinks it is a groove in the underlying tissue or skull bone (not a crease in the skin like a laughter line).
She saw a woman the other day who had the same thing and we have both met a few women over the years with it.
My wife would like to know if anyone here has the same thing, if it will ever go away and what causes it?