Middlesex County Prosecutor Bruce Kaplan announced on Friday the 17-month-old boy became trapped when he pushed his head through an opening between the mattress and plastic mesh siding of his crib.
The prosecutor says pressure to the child's neck restricted the flow of oxygenated blood to his brain.
The Medical Examiner's Office concluded the child died of asphyxia in combination with bronchopneumonia and bronchitis.
Police were called to Maggie's Little School in Perth Amboy in May. The boy was taken to Raritan Bay Medical Center and pronounced dead.
The identity of the child, who lived in Carteret, has not been released.