The National Zoo's panda cub died of a combination of lung and liver problems, zoo officials announced in a press conference Thursday morning.
The six-day-old cub's lungs were not fully formed, according to a necropsy. As a result, the cub's liver didn't get enough oxygen, the zoo said.
The female cub, who lived for almost a week, died Sept. 23. The mortality rate for females born in captivity is 20 percent in the first year, the zoo said.
On Wednesday, keepers began removing the nest that Mei Xiang had made inside the panda house. She is no longer spending much time there, reports News4's Megan McGrath.
As zoo officials prepared to start Thursday's news conference, Mei Xiang was let out into the yard, but kept close to the back fence while her keepers were inside the panda house. After they left, she ventured further into the yard but spent much of her time with her back turned to visitors.
Shortly after the cub's death, keepers said she cradled a toy, a nurturing behavior that she had also shown before the cub's birth. Keepers described this as "expression of her natural mothering instinct."
Preliminary reports had indicated that the cub had fluid in its abdomen and some abnormalities in the liver, including discoloration. A day after the cub's death, Chief Veterinarian Suzan Murray said the free fluid in the abdomen was abnormal for a cub, and could be a symptom of liver problems.
Cubs are born tiny and helpless, at about the size of a stick of butter. The equivalent would be a full-term human baby fitting in the palm of your hand, a display at the panda habitat explains.
Cubs have been known to be accidentally crushed by their mothers, but that was not the case here. The zoo said early on that the cub had no external damage. The cub weighed slightly less than 100 grams, had nursed, and had a coat that was "just beautiful," Murray said Sept. 24.
The odds of Mei conceiving a cub after five consecutive pseudopregnancies since 2007 had been less than 10 percent, the zoo said shortly after the cub's birth. Keepers and panda fans were delighted by the cub's surprise birth on Sept. 16.