Giant pandas are one of the most endangered species in the world and are notoriously hard to breed in captivity.
The National Zoo is one of only four zoos in the US to have pandas, which are on loan from China.
Mei Xiang, who has two other offspring, is one of the zoo's star attractions and a Panda Cam on her enclosure crashed within seconds of the birth of the first cub being announced because of the volume of interest. Female pandas are able to conceive for only two or three days a year, leading to a very low reproduction rate.
Mei Xiang was artificially inseminated with sperm from the zoo's resident male Tian Tian and a panda named Hui Hui from Wolong, China. (s:bbc)