A flat spot on a baby's head.

Plagiocephaly can happen when babies spend time lying on their backs. It doesn't cause brain damage or problems with a baby's growth, but it can be linked to them. It also can be linked to a rare condition in which tight neck muscles cause the head to twist to one side.

The cheekbone and ear on the affected side might look pushed forward. The face can look uneven.

Changing a baby's head position can help. If it doesn't improve by age 4 months or is severe, the baby might need to start wearing a molded helmet before 1 year of age.