Courtesy Zoo Atlanta

A 15-year-old giant panda named Lun Lun, who lives at Zoo Atlanta, is pregnant with her fourth cub.

The new little addition will be the first giant panda cub born in the United States in 2013.

The unborn cub will have three older siblings, male Mei Lan, 6, now a resident of the Chengdu Research Base of Giant Panda Breeding, male Xi Lan, 4, and male Po, 2, all of whom were born to Lun Lun and fellow 15-year-old male Yang Yang.

Watch Lun Lun give birth to baby cub Xi Lan in 2010

The cubs are a product of artificial insemination, just as this one was, when Lun Lun was inseminated in March with the assistance of Dr. Copper Aitken-Palmer, an expert in giant panda reproductive physiology from Smithsonian's Conservation Biology Institute.

Raymond B. King, Zoo Atlanta's President and CEO, said, "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.

"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."

The cub's delivery is expected in about two weeks.