National Zoo staff keeping cautious eye on panda twin cubs in US

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In Surprise Twist, D.C. Panda Gives Birth to Twins


(REUTERS) -- National Zoo staff are keeping a close eye on giant panda Mei Xiang's newborn twins, given the cubs' fragile state just days after their birth, the zoo's lead veterinarian said on Monday.

"The cubs are doing pretty well. These early days everything's still touch and go, so we're watching them closely but we're very hopeful right now," Dr. Brandie Smith told CNN.

Mei Xiang, a star tourist draw, took staff by surprise on Saturday by giving birth to the twins about four-and-a-half hours apart after being artificially inseminated. Giant pandas are among the world's most endangered species.

See photos of the newborns:

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Baby Pandas born at Smithsonian
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National Zoo staff keeping cautious eye on panda twin cubs in US

Mei Xiang in her den with her cub on Sept. 21, 2015. (Photo via Smithsonian's National Zoo)

WASHINGTON, DC - AUGUST 23: In this handout provided by the Smithsonian's National Zoo, the second of two newborn Giant Pandas born August 22 is cared for by members of the panda team at the Smithsonian's National Zoo and Conservation Biology Institute August 23, 2015 in Washington, DC. The team is swapping each cub with the mother Mei Xiang, allowing each to nurse, while the other is bottle-fed and kept warm in an incubator. (Photo by Smithsonian's National Zoo via Getty Images)
WASHINGTON, DC - AUGUST 23: In this handout provided by the Smithsonian's National Zoo, the second of two newborn Giant Pandas born August 22 is cared for by members of the panda team at the Smithsonian's National Zoo and Conservation Biology Institute August 23, 2015 in Washington, DC. The team is swapping each cub with the mother Mei Xiang, allowing each to nurse, while the other is bottle-fed and kept warm in an incubator. (Photo by Smithsonian's National Zoo via Getty Images)
WASHINGTON, DC - AUGUST 23: In this handout provided by the Smithsonian's National Zoo, the second of two newborn Giant Pandas born August 22 is cared for by members of the panda team at the Smithsonian's National Zoo and Conservation Biology Institute August 23, 2015 in Washington, DC. The team is swapping each cub with the mother Mei Xiang, allowing each to nurse, while the other is bottle-fed and kept warm in an incubator. (Photo by Smithsonian's National Zoo via Getty Images)
WASHINGTON, DC - AUGUST 24: In this handout provided by the Smithsonian's National Zoo, one of two newborn Giant Pandas born August 22 is cared for by a member of the panda team at the Smithsonian's National Zoo and Conservation Biology Institute August 24, 2015 in Washington, DC. The team is swapping each cub with the mother Mei Xiang, allowing each to nurse, while the other is bottle-fed and kept warm in an incubator. (Photo by Smithsonian's National Zoo via Getty Images)
WASHINGTON, DC - AUGUST 23: In this handout provided by the Smithsonian's National Zoo, the second of two newborn Giant Pandas born August 22 is cared for by members of the panda team at the Smithsonian's National Zoo and Conservation Biology Institute August 23, 2015 in Washington, DC. The team is swapping each cub with the mother Mei Xiang, allowing each to nurse, while the other is bottle-fed and kept warm in an incubator. (Photo by Smithsonian's National Zoo via Getty Images)

Mei Xiang left her den Saturday, Sept. 5, around 3:10 p.m. to eat and drink. She ate more bamboo than she has in previous excursions since giving birth. Keepers expect Mei Xiang to gradually spend more time away from the den eating and drinking as the cub grows. While Mei Xiang was eating, veterinarians and keepers took the opportunity to give the cub another quick checkup. He weighed 409.6 grams, which was 119 grams more than he weighed on Sept. 2. His 297 percent weight gain is normal for a cub his age. Cubs at this stage usually gain between 40 and 50 grams per day. Veterinarians also listened to his heart and lungs, which all sounded normal. His eyes are still closed, which is normal. Cubs’ eyes generally open when they are 6 to 8 weeks old. He also had a full belly at the time of the exam and appeared to be healthy. (Photo via Smithsonian's National Zoo)

Mei Xiang and her cub on Sept. 3. (Photo via Smithsonian's National Zoo)

Mei Xiang sleeping in her den with her cub Sept. 18, 2015. (Photo via Smithsonian's National Zoo)

Mei Xiang left the den around 9 a.m., about 30 minutes after keepers replenished food in her enclosure on Sept. 12. She spent 15 minutes away from the cub, during which time he could be seen on the panda cam scooting around in circles. Keepers noted that the shape of his back saddle, the black marking on a panda’s back, resembles Tian Tian’s, his father. (Photo via Smithsonian's National Zoo)

Keepers weighed the giant panda cub Sept. 14, when Mei Xiang left her den. He weighed 881.5 grams or 1.9 pounds. (Photo via Erika Bauer/Smithsonian's National Zoo)

At a veterinary exam Sept. 17, the cub measured 31 centimeters from his head to the tip of his tail. From his head to the base of his tail he measured 27 centimeters. His right front leg was 10 centimeters long and his right hind leg was 9 centimeters long. Veterinarians reported that he has an excellent range of motion and is able to push up onto his front legs. Keepers have seen him scooting around the den using his front legs, but he is not walking yet. (Photo via Amy Enchelmeyer/Smithosonian's National Zoo)

At a veterinary exam Sept. 17, the cub measured 31 centimeters from his head to the tip of his tail. From his head to the base of his tail he measured 27 centimeters. His right front leg was 10 centimeters long and his right hind leg was 9 centimeters long. Veterinarians reported that he has an excellent range of motion and is able to push up onto his front legs. Keepers have seen him scooting around the den using his front legs, but he is not walking yet. (Photo via Amy Enchelmeyer/Smithsonian's National Zoo)

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Smith said the new cubs are so tiny and fragile that problems could still arise, anything from a congenital defect to difficulties with the mother panda's ability to care for the two new small babies, each still fur-less and about the size of a stick of butter.

"Almost anything could go wrong in these first few days," she told CNN's "New Day" program. Smith is the zoo's associate director for animal care sciences, overseeing care for all of its mammals.

Zoo staff have been occasionally switching out one cub and keeping it in an incubator, tracking each cub's progress by measuring and weighing them each time. Staff are also continually monitoring Mei Xiang.

The mother panda was artificially inseminated in April with sperm from Hui Hui, a panda inChina, and from the National Zoo's Tian Tian. Zoo officials have said they do not yet know which insemination was successful, and that it is possible the twins have different fathers.

Giant pandas, native to China, have a very low reproductive rate, especially in captivity. There are about 300 giant pandas in captivity and roughly 1,600 in the wild.

Related: Baby pandas revealed to the public in Ya'an, China:

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Newborn pandas in China
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National Zoo staff keeping cautious eye on panda twin cubs in US
YA'AN, CHINA - AUGUST 21: A newborn panda cub is seen at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: Workers care for newborn panda cubs at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: Newborn panda cubs are seen at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: Workers care for newborn panda cubs at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: Newborn panda cubs are seen at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: A newborn panda cub is seen at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: A newborn panda cub is seen at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21:  Visitors look through a window at the newborn panda cubs at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: A newborn panda cub lies in a basket at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: A newborn panda cub is seen at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
YA'AN, CHINA - AUGUST 21: A worker cares for newborn panda cubs at Ya'an Base on August 21, 2015 in Ya'an, Sichuan Province of China. Ten newborn panda cubs born in 2015 are shown to the public in Sichuan's Ya'an Base with the smallest ones less than a week old. The China Conservation and Research Center for the Giant Panda (CCRCGP) has nurtured 21 panda cubs with just one not surviving. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
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