Neither of the boy's two left arms is fully functional and tests have so far been unable to determine which was more developed, said Dr. Chen Bochang, head of the orthopedics department at Shanghai Children's Medical Center.
"His case is quite peculiar. We have no record of any child with such a complete third arm," Chen said in a telephone interview. "It's quite difficult to decide how to do the operation on him."
The boy, identified only as "Jie-jie," also was born with just one kidney and may have problems that could lead to curvature of the spine, according to local media reports.
Jie-jie cried when either of his left arms was touched, but smiled and responded normally to other stimuli, the reports said.
Chen said doctors hoped to work out a plan for surgery, but the boy's small size made it impossible to perform certain tests that would help them prepare.
"We are meeting with several experts now. We hope we could work the plan out soon," Chen said.
Media reports said other children have been reported born with additional arms and legs, but in all those cases it was clear what limb was more developed.
Chen's hospital is one of China's most experienced in dealing with unusual birth defects, including separating conjoined twins. Like Jie-jie, many of the children are sent to relatively wealthy Shanghai from the poor inland province of Anhui.