National Zoo panda Mei Xiang gave birth Saturday afternoon to twin cubs just days after zoo officials confirmed she was pregnant. A few hours later the zoo's panda cam showed her playing with one of the newborns. (Smithsonian National Zoo)

Laurie Thompson was sitting at her desk in the National Zoo’s giant panda house Saturday night when she heard a noise from the video monitor showing Mei Xiang, who had just given birth.

It sounded like Mei was in labor, but she had delivered a cub about four hours earlier. Thompson, a biologist, looked at the monitor, and there, wiggling and squawking on the floor, was a second cub.

Thompson jumped out of her seat, ran into a colleague at the door and yelled, “We have two!”

On Sunday, the National Zoo was trying to cope with the joyous number, and with the task of caring for two cubs the size of large mice, and their mother.

It was only the third time that giant panda twins have been born in the United States. In one of the other cases, the twins did not survive.

The news flashed around the world, and legions of fans stampeded to the zoo’s panda cams.

Keepers quickly began swapping out the cubs so that Mei only had to care for one at a time. The first cub was born at 5:35 p.m. Saturday; the second cub arrived at 10:07 p.m.

The second cub was retrieved by Thompson and keeper Marty Dearie shortly after it was born. It was placed in an incubator, examined, fed three times and returned at 6:30 a.m. Sunday. Then the older cub was retrieved.

Both cubs squealed loudly, a sign of good health. “A screaming baby panda makes us all happy,” said Don Neiffer, the zoo’s chief veterinarian.

Zoo officials said that pandas who have twins often are unable to care for both babies, and one generally dies. The swapping process, which can occur every few hours, increases the likelihood that both cubs will survive.

Thompson said in an interview at the zoo Sunday that she and a few other staffers were staying overnight in the panda house Saturday to keep an eye on Mei and the first cub. There was no hint that a second cub was coming, she said, and many supervisors had left for the night.

“I was sitting next to the computer with the panda cam on it,” she said. “I started hearing this noise, and I was like, ‘That’s the same noise that she was making when she was having contractions.’ ”

Mei Xiang’s cub should follow this basic schedule of panda cub growth.

“As soon as I looked at the camera, out popped the cub,” she said. “I knew she still had one [cub] under her arm, so I knew she had had a second one.”

Thompson ran out of the room, bumped into Dearie and shouted the news. They donned protective suits, booties and gloves in case they had to approach the panda enclosure or touch the cubs.

“We watched her for a few minutes just to see how she was going to do, because if she picked up both of the cubs, we were going to just let her kind of do that for a little bit,” Thompson said. “But she was never able to pick them both up. She was trying, but she just couldn’t figure out how to do it.”

They slipped into an adjacent space with a small door to Mei’s den. As they did, Mei took one of the cubs and retreated, leaving the other on the floor.

Dearie reached into the den and retrieved the cub, Thompson said.

They wrapped the cub in a green towel and carried it to the incubator in the keeper’s office, away from the den. “We don’t want Mei Xiang to hear the cub,” Thompson said. “We don’t want to stress her.”

The cub was squealing. “They squeal until you get them nice and comfortable,” she said. They placed it in the warm incubator, covered it with towels, and Thompson placed her hand on it.

“They like to be snug, just like newborn babies,” she said.

“We were excited, but you’re always nervous because you don’t know if it’s okay,” she said. “But it sounded okay. . . . Because it was squealing and carrying on, we thought, ‘Oh, it's good.’ But we . . . were anxious for everybody to get there.”

As they waited, they allowed themselves a modest celebration.

“We . . . high-fived a little bit, fist bumps, all that,” she said.

When Copper Aitken-Palmer, chief veterinarian at the Smithsonian’s Conservation Biology Institute, arrived, she began calling colleagues at a panda breeding center in Wolong, China.

She speaks a little Chinese, and many of her Chinese colleagues speak English, she said at the zoo Sunday. But she couldn’t reach anyone. So the zoo contacted a friend in California who works as a liaison with China, and she was able to get through.

“We had a bunch of questions that we were mulling around,” Aitken-Palmer said. The Chinese have many more pandas in captivity and much more experience with cubs. She also wanted to share the good news with the Chinese.

Pandas often have twins and sometimes triplets. One set of twins was born at the National Zoo to Ling-Ling decades ago, but both cubs died.

Mei’s second cub was the more robust, weighing about 138 grams, zoo director Dennis Kelly said Sunday. The first cub weighed about 86 grams, he said. Veterinarians were unable to determine the sex of the cubs during their exams.

“We’ve developed a bunch of strategies to be able to swap the cubs, but it’s ultimately up to Mei Xiang and how cooperative she is with us in the whole swapping process,” Kelly said. “Obviously, these are her cubs and she’s not too keen on us taking them from her.”

The cubs’ eyes are not open and their ears are small “nubs,” Thompson said. They have fine white fur and distinct claws.

Mei has delivered two surviving cubs since 2005 — Tai Shan, a male, born on July 9, 2005, and Bao Bao, a female, who celebrated her second birthday at the zoo Sunday before a large crowd of visitors.

Tai Shan lives in China, which owns and leases all giant pandas in U.S. zoos.

Mei also gave birth to a stillborn cub about 26 hours after Bao Bao in 2013. And on Sept. 16, 2012, she gave birth to a cub that survived less than a week because of liver abnormalities.