The National Zoo in Washington DC says its adult female panda has had twins.
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.
Mei Xiang's first cub, Tai Shan, was born in 2005 and returned to China in 2010. Her second cub, Bao Bao, is two years old today and still lives at the zoo.
Keepers will be watching the new cubs closely. Pink, hairless and blind, newborn cubs weigh just three to five ounces.
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. But the Washington pandas have a history that makes them closely watched.
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. The pair had five cubs while living at the zoo but none survived.
Mei Xiang and Tian Tian, the parents of both Bao Bao and Tai Shan, arrived in 2000. The pandas belong to China as do any cubs they have.