BACKGROUND The physiological basis underlying the decline in the incidence of sudden infant death syndrome (SIDS) associated with changing the sleep position from prone to supine remains unknown.

AIMS To evaluate diaphragm thickness (t_di) and shortening in healthy term infants in the prone and supine positions in order to determine whether changes in body position would affect diaphragm resting length and the degree of diaphragm shortening during inspiration.

RESULTS At end expiratory (EEV) and at end inspiratory lung volumes (EIV), t_di increased significantly in the prone position. The change in t_diduring tidal breathing was also greater when the infant was prone. Sao ₂, HR, and f were not significantly different at EEV and at EIV in both positions.

CONCLUSION In healthy term infants, placed in the prone position, the diaphragm is significantly thicker and, therefore, shorter, both at EEV and EIV. Diaphragm shortening during tidal breathing is greater when the infant is prone. In the prone position, the decreased diaphragm resting length would impair diaphragm strength, and the additional diaphragm shortening during tidal breathing represents added work performed by the diaphragm. This may compromise an infant's capacity to respond to stressful situations when placed in the prone position and may contribute to the association of SIDS with prone position.