Age-dependent changes in infant head shape: a smartphone app-based study in Japan

Head shape undergoes rapid changes during infancy, but the age-dependent changes in cranial asymmetry remain inadequately characterized. This study aims to analyze factors associated with these parameters, and to establish reference curves in Japanese infants. Two indices of head shape, the cranial vault asymmetry index (CVAI) and cranial index (CI) were collected from 127,605 Japanese infants via a smartphone app. We performed univariate and multivariate analyses to identify factors associated with CVAI and CI. Using 72,726 data derived from infants born via spontaneous vaginal delivery with normal birth weight, reference curves for CVAI and CI were constructed. CVAI exhibited a peak at 3-4 mo of age, followed by a gradual decline, whereas CI increased until approximately 6 mo of age. Boys had significantly higher CVAI than girls. Low birth weight was associated with low CI. Reference curves revealed distinct age-specific patterns, with CVAI peaking at 3-4 mo and declining thereafter, and CI increasing until around 6 mo. In conclusion, this study provides the first reference curves for CVAI and CI that can be applied for Japanese infants. The findings highlight the natural course of cranial asymmetry and proportionality, emphasizing the importance of age-specific assessment.