A group of artificially bred panda cubs were put together on Monday, for the first time since their births in the city of Chengdu in southwest China's Sichuan Provinces. (Sept. 23)

What’s cuter than a panda cub? Fourteen cubs sharing an oversized crib at the Chengdu Research Base of Giant Panda Breeding in China.

Researchers wearing blue surgical gowns, hairnets and face masks presented the furry cubs to the world Tuesday as they gave an update on the recent panda baby boom. A total of 20 cubs were born to pandas from the research base between July 10 and Aug. 30, according to Huang Xiangming, chief of the animal management division there.

Seventeen of those cubs are still alive, Huang told the China Daily newspaper. Two of those cubs are twins born at Zoo Atlanta on July 15 and are doing well (you can follow their progress on the zoo’s blog, which includes a webcam). Another cub was born in Spain.

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The remaining 14 cubs are at the research base in Chengdu, capital of China's Sichuan province. The biggest weighs nearly 9 pounds and the smallest is only 1.5 pounds, according to the BBC.

The sight of 14 cubs in one place is striking not just for its overwhelming cuteness but because pandas are notoriously difficult to breed in captivity. (It’s not so easy for them to breed in their natural habitat either -- only about 1,600 are left in the wild, according to the most recent estimate from the World Wildlife Foundation.)

PHOTOS: Panda cubs meet the press

Among the difficulties is the fact that “pandas are generally solitary as adults,” according to the San Diego Zoo, which has a breeding program for giant pandas. If a female is getting ready to go into heat, she leaves scent marks to alert any males who happen to be passing by. But those males had better be in the right place at the right time -- female pandas are “receptive to breeding” for only two or three days a year, the San Diego Zoo says.

In captivity, it’s a simple matter for keepers to get males and females together when the time is right. The panda pair in San Diego has conceived five cubs the old-fashioned way. But sometimes the pandas don’t know what to do next.

At the Chiang Mai Zoo in Thailand, pandas Chuang Chuang and Lin Hui seemed to be at a loss, so handlers showed Chuang Chuang (the male) some panda porn, according to a 2006 report from National Geographic. The zoo’s Prasertsak Buntrakoonpoontawee explained to Reuters that "they don't know how to mate, so we need to show the male how through videos," the NatGeo story says.

In zoos, breeders often resort to artificial insemination to speed along a birth. A male giant panda is given a drug like propofol, ketamine or another anesthetic during the collection process. (This website from the University of Wisconsin’s Department of Animal Sciences explains the procedure in great detail.) Then the female is sedated for the insemination procedure. All five cubs born in Atlanta -- including the new twins -- are products of artificial insemination, according to National Geographic.

It can take about two months to know whether the the procedure worked. A panda may look like she’s pregnant, act like she’s pregnant, and have hormone levels consistent with pregnancy -- but it may all turn out to be a false alarm. Veterinarians call this a pseudopregnancy, and it also happens to other mammals, including mice, dogs and sometimes people, according to Suzanne Hall of the San Diego Zoo’s Institute for Conservation Research.

If the pregnancy is real, there can be a delay of as much 100 days as the embryo waits for the right time to implant itself in the uterus, Hall wrote. This period, known as embryonic diapause, begins after the fertilized egg goes through just a few rounds of cell division, Hall wrote. Scientists don't know why pandas (and about 100 other mammals) experience embryonic diapause or what causes it to end. The length of the delay can vary from pregnancy to pregnancy, keeping breeders in the dark about the success of their efforts.

But sometimes everything comes together. More than 100 panda cubs have been born at the Chengdu Research Base.

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