Developmental milestones refer to skills that children may acquire over the course of their lifespan. Typically, a skill first emerges and is demonstrated inconsistently until a child masters it, at which point it is considered “achieved.” In the practice of pediatrics, familiarity with typical developmental progression, with the age at which 75% of children typically acquire a skill, and the ability to recognize which children need additional intervention to reach a milestone, are essential competencies. This proves more difficult in a population that almost universally develops differently due to a specific condition, such as those with Down syndrome.