I can answer this one as coincidentally Amelia had an appointment with her plastic surgeon today and he notice her birthmarks and said he would treat them aswell. He told us to keep an eye on them and if they start bleeding phone him and hell oversee treatment. I think the worry about it being on her bum is that she lies on it and will eventually sit on it. He said this is why he wants to keep a close eye on Amelias as it on the back of her head, so whenevr shes on her back shes lying on it. Amelias has always been fine though
So you only need to worry really if it starts bleeding or looking sore, youll know if something is wrong x