The zoo announced Lun Lun's pregnancy was confirmed during a June 30 ultrasound and the size of the fetus indicates it could be born within the next two to three weeks.
The zoo's animal management and veterinary teams said the cub will be the first giant panda born in the United States in 2013.
"We're thrilled about the possibility of a fourth cub for Lun Lun, but we remain cautiously optimistic at this point. Giant panda cubs are extremely fragile, and the chance remains that the fetus does not go to term," Raymond King, the zoo's president and chief executive officer, said. "A birth would be another success for our giant panda program, and we share our optimism with our fellow U.S. zoos housing this iconic species and with our colleagues at the Chengdu Research Base of Giant Panda Breeding in China."