APA Dictionary of Psychology

the significant achievements in motor development that occur during an infant’s first 2 years. Although individual children vary, on average infants will (a) support their head while prone at 3 months; (b) support their head in other positions at 4 months; (c) sit with props at 5 months; (d) sit supported by their hands and reach with one hand at 6 months; (e) pick up small items, using thumb opposition, and stand if holding onto a railing at 8 months; (f) creep, pull to a standing position, and take side steps while holding onto a support at 10 months; (g) walk alone, throw a ball, and walk backwards, sideways, upstairs, and downstairs with assistance at 16 months; and (h) run and walk up and down steps easily or with minimal assistance at 24 months. See also developmental sequence.