Giant panda Mei Xiang gave birth to a second cub Saturday night, hours after having the first cub, the National Zoo in Washington, D.C., said.
The second panda was born at 10:07 p.m., the zoo said in a statement. One of the cubs has been taken by specialists and placed in an incubator, and "it appears to be in good health," the statement said.
Mei Xiang gave birth to the first cub at 5:35 p.m., about an hour after the zoo said Mei Xiang's water had broken. After the birth, Mei Xiang reacted to the cub by picking it up.
"All of us are thrilled that Mei Xiang has given birth. The cub is vulnerable at this tiny size but we know Mei is an excellent mother," zoo director Dennis Kelly said in a statement.
"Thank you to all of our excellent keepers, veterinarians, researchers and Chinese colleagues who contributed and therefore deserve credit for this conservation success," he said.
The zoo's panda team are monitoring Mei Xiang and the cub via cameras, and will perform a neonatal exam in a few days, the zoo said.
Earlier, the zoo had said it appeared Mei Xiang was in labor.
"She is restless, having contractions, body licking and remains in her den — all behaviors consistent with an imminent birth," the Washington, D.C. zoo posted on its Facebook page Saturday. "We are monitoring Mei very closely through the panda cams and are hoping for a healthy cub."
The zoo shared a link to its panda cam for spectators to watch the event — but the live video appeared to be down due to high traffic, much to the chagrin of panda fans.
"Please have them turn the cams back on. I would love to witness this delivery," one wrote on the zoo's Facebook page.
Determining whether or not a panda is pregnant is surprisingly difficult. Mei Xiang was artificially inseminated in April with semen from Hui Hui, a giant panda at the China Conservation and Research Center for the Giant Panda, and with semen from the National Zoo's male giant panda, Tian Tian.
It was not until this past week that veterinarians detected what they believed to be a fetus on an ultrasound.
"Based on the size of the fetus, which is about four centimeters (1.6 inches), veterinarians estimate that Mei Xiang could give birth early next week, or possibly in early September," the zoo said in a statement.
Mei Xiang, whose name means "beautiful fragrance," according to the zoo, has two cubs: Bao Bao, born in August 2013, and Tai Shan, born July 2005.
With just over 1,800 giant pandas in the wild, pandas are one of the world's most endangered creatures, according to WWF.
Mei Xiang has had five "false pregnancies," where she showed symptoms of expecting, but had no cub inside of her.
According to the zoo, Mei Xiang weighs 233 pounds. At birth, a giant panda baby typically weighs three to five ounces — about one nine-hundredth the size of its mother. Except for marsupials, such as kangaroos or opossums, that's the smallest mammal newborn relative to its mother's size, the zoo says.