External deforming forces: Positional plagiocephaly occurs secondary to mechanical deformational forces applied to the infant’s growing skull, which is softer and more pliable than the adult skull.
Development in utero or in postnatal period: Positional plagiocephaly can occur in the prenatal or postnatal period. Evidence suggests that infants with positional plagiocephaly are less active (14) and are delayed in some areas of development compared with age-matched controls (7,16). There is no proven “cause and effect,” and it is likely that infants with developmental delay spend more time in the supine position compared to those without delay.
Associated with supine positioning: Positional plagiocephaly most typically occurs in relation to supine positioning.
Parallelogram deformation: A unilateral occipitoparietal flattening can be accompanied by anterior displacement of the unilateral ear, contralateral occipitoparietal bossing, ipsilateral frontal bossing, and contralateral frontal flattening (see illustration under “Evaluation”). These changes often give the head a characteristic parallelogram shape when viewed from the vertex (35).