I'm not sure if this is serious and I'm worried

What does her pediatrician advise?

She measured it and said it usually goes away when they are 3 or 9 I'm worried and I was reading it can grow and become worst

You'll be taking your baby to the doctor for regular well-baby checkups and for inoculations, so the doctor will keep an eye on it. Do not mess with it and poke around on it.