Atlanta is expecting the first giant panda born in the U.S. this year.

Officials at Zoo Atlanta said Tuesday that one of the zoo's giant pandas is pregnant with her fourth cub. Fifteen-year-old Lun Lun will likely deliver a cub in the next two to three weeks as veterinarians based their birth window estimate on a similar ultrasound from Lun Lun's last pregnancy in 2010.

"We're thrilled about the possibility of a fourth cub for Lun Lun," said zoo president and CEO Raymond B. King. "But we remain cautiously optimistic at this point."

An ultrasound done Sunday confirmed the pregnancy but King said giant panda cubs are extremely fragile and a chance remains that the fetus might not make it to term.

The zoo said hormone tests being performed by an expert in giant panda endocrinology from Western University of Health Sciences, will allow the veterinary team to continue to monitor her condition.

"A birth would be another success for our giant panda program," said King, "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."

Lun Lun was artificially inseminated in March and the ultrasound showed that the fetus is currently 1.08 centimeters long. She and her mate, Yang Yang, also 15, already have a 4-year-old male named Xi Lan and a 2-year-old male named Po with them at the Atlanta facility. Their eldest offspring, a 6-year-old male named Mei Lan, now lives at the Chengdu facility.