The first cub was born on Saturday at 5.35pm local time and the zoo said that a second was born at 10.07pm, and both cubs appear healthy.
If the cubs survive, they would be the 17-year-old panda’s third and fourth surviving offspring.
Keepers will be watching the new cubs closely. Pink, hairless and blind, newborn cubs weigh just three to five ounces.
After the first birth, Pamela Baker-Masson, the zoo’s spokeswoman, said the staff were “thrilled, absolutely thrilled.”
“I’ve been in close communication with veterinarians, the scientists, keepers. Everybody’s extremely happy,” she said. “We were all tuned in to the panda cam, and we saw her water break. And then just about an hour later . . . she gave birth to a cub.”
One of the two was retrieved, following a special protocol for rearing twins, another zoo spokeswoman said. It was placed in an incubator. It was not clear which of the two cubs it was.
However, the zoo said, both cubs would be given the opportunity to bond with their mother.
The zoo’s “panda team” would alternate the cubs between maternal and incubator rearing.
One will be nursing and spending time with the mother, while the other would be fed by bottle and kept warm in the incubator. Authorities were alerted to the second birth through watching a video feed of the panda habitat.
It occurred quickly, more rapidly than the day’s first birth, Ms Baker-Masson said.
The loud noises from the first cub after birth were a “great sign for good health,” said Ms Baker-Masson, who added that staffers watched with awe as the mother picked up her cub.
Inside the panda house on Saturday afternoon, a small, rapt knot of onlookers watched on a video monitor as Mei Xiang tossed and turned in her cage. About 4.30pm, zoo officials confirmed that the panda’s water had broken.
At 5.35pm, the first glimpse of the baby came on the screen. People burst into applause and shouted, “Yes! Yes! Oh, my God!”
Mei Xiang, who went into labour about 10.30am on Saturday, has already delivered two surviving cubs since 2005 — Tai Shan on July 9, 2005, and Bao Bao on August 23, 2013.
She gave birth to a stillborn cub about 26 hours after Bao Bao. And on September 16, 2012, she gave birth to a cub with liver abnormalities. It died six days later.
The zoo has also had disappointments in the past. Mei Xiang gave birth to a stillborn cub in 2013. And in 2012, she gave birth to a cub that died after just six days. Its lungs had not fully developed.
Even if the new cubs are healthy, panda fans should not expect to see them in person for a while. After Bao Bao was born, it was about five months before she made her public debut.
Fans who want to see the newest pandas will have to try to catch a glimpse of them on the zoo’s online panda cam.
The public also will not learn immediately whether the cubs are male or female or whether the zoo’s male panda, Tian Tian, is their father.
It takes time to determine a tiny cub’s gender and Mei Xiang was artificially inseminated with sperm from Tian Tian and a panda named Hui Hui from Wolong, China, who was thought to be one of the best genetic matches.
The National Zoo is one of only four zoos in the US to have pandas, which are on loan from China. The zoo’s first pair of pandas, Ling-Ling and Hsing-Hsing, were a gift from China following President Richard Nixon’s historic 1972 visit to the country.