Brown was fatally shot by a police officer August 9 in Ferguson, touching off a week of rancorous protests in the St Louis suburb.
Shawn Parcells, a pathologist hired by Brown's family, said a bullet wound to the teen's arm may have happened when he put his hands up, "but we don't know".
Parcells said a graze wound on Brown's right arm could have occurred in several ways. He says the teen may have had his back to the shooter, or he could have been facing the shooter with his hands above his head or in a defensive position.
The autopsy was conducted by former New York City chief medical examiner Dr Michael Baden. He found that one of the bullets entered the top of Brown's skull, suggesting his head was bent forward when he suffered the fatal injury.
Baden has testified in several high-profile cases, including the OJ Simpson trial.