Intra-individual stability of rate of gross motor development in full-term infants.

The purpose of this study was to evaluate the intra-individual stability of gross motor scores obtained by normally developing full-term infants on the Alberta Infant Motor Scale (AIMS). The gross motor skills of 47 infants were assessed monthly in their homes by pairs of physical therapists. Infants were followed from two weeks of age until they achieved independent walking. A developmental pediatrician assessed each infant at 18 months of age, and classified the infant's gross motor skills as normal, suspicious or abnormal. Only the data of infants receiving a normal classification at 18 months were included in the analyses (n = 45). Individual infants' percentile ranks varied considerably from month to month, with no systematic pattern of change noted across infants. As a group, the mean percentile range over 13 assessments was 66.78 (S.D. 13.47). Fourteen infants (31.1%) received a score below the 10th percentile on at least one occasion. The results suggest that normally developing infants are not stable in the rate of emergence of gross motor skills. This instability has implications for infant screening programs, and supports the premise of serial assessments to identify accurately those infants with a motor delay.