Lun Lun, a 15-year-old giant panda at Zoo Atlanta, has been confirmed to be expecting her fourth cub. Based on the size of the fetus observed in an ultrasound conducted on June 30, 2013, the Animal Management and Veterinary Teams estimate that a birth could take place in two to three weeks. The cub would be the first giant panda born in the U.S. in 2013.

Veterinarians base their assessment of Lun Lun’s birth window on data from a similar ultrasound performed when Lun Lun was expecting her third cub, Po, in 2010. The fetus is now 1.08 centimeters long, which is consistent with a delivery date approximately two weeks from now.

Over the past several days, Lun Lun has been exhibiting behavioral signs indicative of either pregnancy or pseudopregnancy. Hormone assays conducted by Dr. Dave Kersey, an expert in giant panda endocrinology from Western University of Health Sciences, will continue to shed light on her condition.

“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,” said Raymond B. King, president and CEO. “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.”

Lun Lun was artificially inseminated in March 2013 with the assistance of Dr. Copper Aitken-Palmer, an expert in giant panda reproductive physiology from Smithsonian’s Conservation Biology Institute. Lun Lun and 15-year-old male Yang Yang have three previous offspring: male Mei Lan, 6 and now a resident of the Chengdu Research Base of Giant Panda Breeding; male Xi Lan, 4; and male Po, 2. All three were the products of artificial insemination, and all three were the only giant pandas born in the U.S. in their respective birth years.