The mother Asian elephant, Maharani, was reportedly calm before delivering the dead male calf early Wednesday, the Calgary Herald reported.
The zoo's director of animal care, Dr. Jake Veasey said in a release a preliminary necropsy showed the calf had been dead for several weeks.
Grief counselors were called in for the elephant handlers, the report said.
Elephants have a 22-month gestation period and the calf wasn't due until February at the earliest, the Calgary Sun said.
This was the third loss of a calf for Maharani. She rejected her first calf and her second died of elephant herpes virus at the age of 15 months, the Sun said.
Zoo officials said they intend to keep the elephant in its breeding program, but Zoocheck Canada spokeswoman Julie Woodyer called the move irresponsible and "just to replenish zoos so people can look at elephants."
The Calgary Zoo has been exploring potential sites to send their one male and three female elephants, but has said it would only consider facilities with breeding programs.