Suzan Murray said Monday it was too early to tell if the abnormalities were factors in the cub's death,The Washington Post reported.
"Judging too much on the initial necropsy can be a little bit dangerous at times," Murray said. "Something that might appear abnormal to you visually, when you look at it under a microscope it may indeed be normal."
Murray said lab tests performed on tissue samples could help identify the cause of death.
The panda cub died Sunday morning, officials said.
Zoo staff members heard the cub's mother, Mei Xiang, making distress sounds and knew something was wrong.
Zookeepers attempted to revive the panda but were unsuccessful.
It was the first birth of a giant-panda cub at the zoo since 2005, the newspaper said.