Butterfly vertebrae are vertebrae (bone that make up your spine) that have a cleft, an indentation or opening, through the middle. If you look at it with an x-ray, the shape reminds you of a butterfly. A human spine is formed within the first few weeks of gestation. So, as the spine is forming, somewhere between weeks three and six of gestation, something goes wrong in the spine and the result is butterfly vertebrae. Usually, this doesn’t cause any problems and goes undetected unless there is a reason to x-ray that part of the back.