To describe writhing General Movements Assessment (GMA) classification and General Movement Optimality Score–Revised (GMOS-R) profiles in the general population; to explore relationships between GMOS-R scores and GMA classification, age of assessment and infant socio-demographic factors; and to establish the inter-rater reproducibility of writhing age GMA classification and GMOS-R.

A cross-sectional study of 1861 infants recruited from the general population in Perth, Western Australia, including 7.5% born preterm. Parent-recorded videos were collected between 41- and 45-weeks post-menstural age, when writhing movements are typically observed. General movements (GMs) classification and GMOS-R were assessed independently by at least 2 experienced advanced-trained clinicians.

The majority of infants' GMs were classified as normal (66.8%) or poor repertoire (33.2%), with one infant's movements classified as cramped synchronized (0.1%). The median GMOS-R score was 33 (interquartile range 27-36). The GMOS-R differentiated GMA classifications. Lower gestational age, any nursery admission, minority ethnicity, and older age at video collection were each associated with a small but statistically significant reduction in GMOS-R scores. GMA classification had excellent inter-rater reliability and agreement. Total GMOS-R had excellent inter-rater reliability and moderate agreement.

Although most infants had normal GMs there was a high prevalence of poor repertoire GMs in the general population. GMOS-R scores effectively differentiated movement quality within the poor repertoire classification. This study provides population-based percentile ranks for interpreting individual post-term GM assessments and as a reference for future research.