Motor and language abilities from early to late toddlerhood: using formalized assessments to capture continuity and discontinuity in development.

Developmental tests reflect the premise that decreases in skills over time should be a sign of atypical development. In contrast, from a psychological perspective, discontinuity may be viewed as a normal part of typical development. This study sought to describe the variability in patterns of continuity and discontinuity in developmental scores over time. Seventy-six toddlers (55% boys) from a larger screening study were evaluated at 13 and 30 months using the Mullen Scales of Early Development (MSEL) in five areas: gross motor, fine motor, visual perception, receptive language, and expressive language. Parents completed the First Year Inventory (FYI) at 12 months as well. At 30 months, 23.68% of the sample received a clinical diagnosis (e.g., developmental delay, autism spectrum disorder [ASD]). Toddlers were classified as stable, increasing, or decreasing by at least 1.5 standard deviations (SD) on their scores in each of the five MSEL areas from 13 to 30 months. Between 3.9% and 51.3% of the sample was classified as increasing and 0-23.7% as decreasing across areas. Decreases in motor areas were associated with increases in language areas. None of the toddlers showed decreases greater than 1.5 SD on their MSEL composite scores. There was no single pattern that characterized a certain diagnosis. Higher FYI sensory-regulatory risk was associated with decreases in gross motor. Lower FYI risk was linked with increases in receptive language. Developmental discontinuity in specific developmental areas was the rule rather than the exception. Interpretations of decreases in developmental levels must consider concurrent increases in skill during this emerging period.