WASHINGTON • An artificially inseminated giant panda took United States zoo officials by surprise when it gave birth to twins more than four hours apart.
Mei Xiang, a star draw in Washington's National Zoo, gave birth to its first cub on Saturday after its water broke about an hour earlier, zoo officials said.
"All of us are thrilled that Mei Xiang has given birth," zoo director Dennis Kelly said in a statement before the second cub arrived.
"The cub is vulnerable at this tiny size but we know Mei is an excellent mother."
Mr Kelly told a news conference that zoo officials were being cautious and "keeping their fingers crossed" after the zoo lost a six-day-old cub in 2012.
"This is still a very fragile time for this cub," he said of the first arrival, which chief veterinarian Don Neiffer said was showing healthy signs, including vocalising.
Mei Xiang picked up the cub soon after giving birth. Zoo officials plan to leave the mother and its cub alone for as long as possible.
"We're taking a very hands-off approach," Mr Neiffer said.
"I'm very much in favour of mum and baby having time together."
About 4-1/2 hours after the first cub was born, the second cub arrived in a surprise delivery.
One cub was placed in an incubator, in line with protocol when twins are born.
Mei Xiang has previously given birth to two surviving cubs, Tai Shan in 2005 and Bao Bao in 2013.
Mei Xiang was artificially inseminated on April 26 and 27 with frozen sperm from Hui Hui, a panda in China, and fresh sperm from the National Zoo's Tian Tian.
Zoo veterinarians first detected evidence of a foetus during an ultrasound scan last Wednesday.
Mr Kelly said the gender of the cubs, or which of the sperm donors is the father, will be determined later. He added that no decision has been made about naming the cubs.
Giant pandas, one of the world's most endangered species, are known for their striking black and white markings .
With a very low reproductive rate, particularly in captivity, their natural home is in a few mountain ranges in central China.
There are about 1,600 giant pandas known to be living in the wild and some 300 in captivity, mostly in China.