My son has a dimple above his buttocks. What is this?

Q: My son was born with what looks like a pinhole above the crease of his buttocks. His doctor says it’s a harmless dimple. How common is this?

A: Very common: At least 1 in 50 babies has these dimples. In fact, I see them every few months, and my advice is always the same: Do nothing at all, but watch for any discharge or swelling. If you do notice either of these, your doctor will want to make sure your child doesn’t have an infection or that a cyst isn’t beginning to form. If, on the other hand, your baby has a large dimple that is unusually colored, or that has any drainage or seems to be tender shortly after birth, your doctor will need to order tests immediately—likely an ultrasound and/or MRI. She’ll want to see if there’s a connection of any kind to the spinal cord, such as spina bifida. This is extremely rare, however. Virtually all of these dimples are harmless pits with no known cause, and no tests or treatment are needed. They develop in utero and usually go away in the first few years.