Ok, here it is. I showed a tiny bit of her butt crack to help show where the birth mark is.
Btw, she was cracking up that I was taking a photo of her butt.
That's a dear little mark. I'm not a dr so I don't know it's it's an indicator of anything, but I am a mom and my son has more then of few that look exacly like that one. Two are on his spine, a large one on the base. They have faded and changed shape over the years. I can assure you that he is heatlhy and able bodied. Two of our other sons also have this marks on their spines as wel.