(CNN) -- A day after a jubilant first delivery, giant panda Mei Xiang gave birth to a second cub Saturday night, but it was stillborn.
Washington's celebrity panda gave birth to the first cub Friday, to the excitement of hordes of fans watching it on public panda cams.
The cub, about the size of a stick of butter, appeared to be doing well. So was its mom, who was cradling it in her den at the Smithsonian's National Zoo.
"It is robust, healthy and fully formed," the zoo said in a Twitter post Sunday after the cub's first neonatal exam.
After the first cub's delivery, zoo officials said there was a 50% chance of a twin being born by Saturday.
But the zoo's second baby watch did not end so well. Mei Xiang's second cub arrived at 7:29 p.m. Saturday.
She groomed it for 17 minutes before it fell onto the floor of the den, said Pamela Baker-Masson, a spokeswoman for the zoo.
"It lay motionless and made no sound. Throughout, staff could see it visually and hear the first cub squealing, and Mei never set it down," she said in a statement.
After zoo staff used a device to grab the motionless cub, they realized it had developmental abnormalities and wasn't fully formed.
"It was never alive. A necropsy is under way, and the zoo will provide additional information tomorrow," she said.
'Not going to relax'
The first cub is doing well, and the zoo is cautiously optimistic it will thrive.
"The cub is nursing & digesting well. It is very active, very vibrant & very pink," the National Zoo said Sunday.
In another post, the zoo said veterinarians observed that the cub's heart rate was steady and its belly was full.
"They could hear breath sounds from both lungs," the zoo said.
A year ago, Mei Xiang delivered a female cub that died within days because of lack of oxygen due to underdeveloped lungs.
"After our last experience, and this is such a small cub, I am not going to relax," said Dennis Kelly, the zoo's director. "We're going to be tense for the next two or three months. We have high hopes."
It will take two to three weeks to know the sex of the cub and zoo officials won't name it for 100 days, following Chinese tradition.
The cubs were conceived through artificial insemination, and it was the third pregnancy for Mei Xiang, 15. The National Zoo says the father is either their own Tian Tian, 15, or the San Diego Zoo's Gao Gao, who is about 23.
The National Zoo said it also expects to know the cub's paternity in two to three weeks.
All three pandas are on loan from China.
Mei Xiang and Tian Tian are already the parents of Tai Shan, who was born in 2005 and is now in China, the native region for the endangered animals.
American zoo officials are consulting with their Chinese counterparts about panda reproduction and ways to encourage newborns to thrive in captivity.
The giant panda is one of the world's most endangered species, with an estimated 1,900 in existence.
America welcomed its first panda twins in 26 years in July at Zoo Atlanta. The twins were the first for Lun Lun, who has two other offspring at that zoo, and were the product of artificial insemination as well. Their father is Yang Yang, also a resident at Atlanta's zoo.
National Zoo experts began watching Mei Xiang a couple of weeks ago. The anticipation peaked when she started cradling objects.
CNN's Devon Sayers contributed to this report.