A sacral dimple or pilonidal dimple is a small hole located just above the buttocks. The name comes from the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

, the bone at the end of the spine, over which the dimples are found. More than 2% of babies may have such dimples; almost all of these are very minor. However, it may be a sign of Spina bifida

Spina bifida

Spina bifida is a developmental congenital disorder caused by the incomplete closing of the embryonic neural tube. Some vertebrae overlying the spinal cord are not fully formed and remain unfused and open. If the opening is large enough, this allows a portion of the spinal cord to protrude through...

: even so this is usually the spina bifida occulta form which is the least serious kind. Sacral dimples are usually spotted in post-natal checks by a pediatrician, who will check:

• Can the floor of the dimple be seen to be covered with skin? If not, it may be that the neural tube is not completely closed.
• Is there a tuft of hair in the dimple? This may also indicate problems.
• Are there any other problems in the examination of the baby, such as weak lower limbs.
• How close to the buttocks is the dimple? The lower, the better.

Additionally, this can be indicative of a possible kidney problem, which is then checked by an ultrasound.