My baby has a dimple on the base of her spine that was there at birth. can that be occult spina bifida?

No: Spina bifida occulta is by definition occult, meaning there are no symptoms and is not a serious condition. Depending on where the dimple is, this could represent other more serious spinal problems. See your doctor.

Unlikely,but: The normal formation of the lower spine can include a small blind ended dimple. If there were a hairy tuft or some worry that it went too deep or connected to the inside, an MRI under anesthesia could identify a problem. I have never found one that I wanted to go through this level of testing in more than 3 decades of practice, but I consider it.