Cognitive/communicative functioning typical of children half of the child’s chronological age.
Motor development typical of children half of the child’s chronological age.
Development or function typical of children only two-thirds of the child’s chronological age in two or more areas (that is, social, motor, and/or cognitive/communicative).
Failure to sustain ongoing, reciprocal social interaction, as evidenced by either:
inability by six months to participate in vocal, visual, and movement-based social exchanges (for example, trading facial expressions)
failure by nine months to communicate basic emotional responses, such as cuddling or exhibiting protest or anger, or
failure to attend to a caregiver’s voice or face or to explore an inanimate object for a period of time appropriate to the infant’s age.