A July 21 Metro article incorrectly said that there has not been a successful giant panda birth in the United States via natural mating. There was such a birth at the San Diego Zoo in 2003. (Published 7/22/2005)

Zookeepers in Atlanta and San Diego are hopeful their giant pandas are pregnant, which would create a record U.S. baby boomlet of the rare species that began this month with the birth of a cub at the National Zoo.

The possibility of three U.S.-born cubs in the same year reflects the coming of age of the giant pandas on loan from China in the past several years as well as a burst of scientific knowledge on producing and caring for their offspring. Artificial insemination techniques are more precise; pregnancy tests are more accurate; and the odds of a fragile newborn surviving are improving dramatically.

Cub births, and survival, have also risen in China. Advances have been especially notable among twins; whereas one newborn was often doomed in the past, now both routinely make it to adulthood.

At the National Zoo, the 12-day-old cub has developed darkened skin around its eyes and on its ears, back and hind legs, the beginnings of the distinctive giant panda appearance that will be completed when its dark fur grows in.

In Atlanta this week, zookeepers began a round-the-clock birth watch, reporting that the zoo's artificially inseminated female, Lun Lun, is eating less, resting more and showing other signs of pregnancy -- or the false pregnancy for which pandas are notorious. In San Diego, the zoo's top panda expert said yesterday there are "hopeful signs" that two-time mother Bai Yun is pregnant.

"It's fantastic to think we've gone in a decade from having no births to potentially having births in three different institutions," said Lisa M. Stevens, the National Zoo's assistant curator for giant pandas. "It's inspiring in the sense that you can take a critically endangered species and make the kind of progress we have made in a decade."

The National Zoo's cub remains in its mother's arms most of the time, though Mei Xiang left it three times in the last five days to get a drink of water in the next room, Stevens said. The cub has been quiet in those times, and Mei Xiang might be preparing her newborn to spend more time on its own.

"We have food strategically placed so she practically has to walk on [the food] to get to the water," Stevens said, but the mother panda has not eaten since the birth. Keepers hope she will get something to eat and leave her cub long enough to allow veterinarians to examine the cub's vital signs and determine its sex. "We're basically following Mei Xiang's lead," Stevens said.

So far, zoo staffers have seen the cub only through the cameras in the birthing den that are also connected to the Internet -- the same images that people worldwide have been able to view. Stevens said keepers could "see the little claws on its feet" earlier this week. The Panda House is closed to the public for at least three months, but the yard is open, and male Tian Tian can sometimes be seen there.

Panda newborns are tiny and vulnerable, but Stevens said she is encouraged by signs that Mei Xiang is developing into a reliable, tender mother. The cub is fattening up and acquiring antibodies from its mother's milk, she said.

"Every day that passes, I feel more comfortable, but not until it looks like a panda and it's eating bamboo -- then I will really begin to relax," she said.

Don Lindburg, the San Diego Zoo's panda conservation team leader, credits the current breeding success in part to active information trading on insemination techniques, hormone monitoring and other pregnancy-related topics. "Now we are moving into a phase where, hopefully, we can see a lot of collaboration on infant rearing," he said.

San Diego's Bai Yun and mate Gao Gao, both 13, mated in early April.

If Bai Yun is pregnant, hers would be the first cub born in the United States as a result of natural mating. The others were achieved through carefully timed artificial insemination, which must be done during the two-day annual window of opportunity during which the female panda is in heat.

"We have some very hopeful signs that come from what we are seeing up to this point, but we are not yet at the point of being able to make an announcement," Lindburg said. "That could come in the near future."

At Zoo Atlanta, Lun Lun and mate Yang Yang tried and failed to mate, so the female was artificially inseminated March 22. She has been nesting and showing other signs that indicate pregnancy since early this month.

Rebecca J. Snyder, curator of giant panda research and management at Zoo Atlanta, said she sends weekly panda urine samples to the National Zoo, where they are analyzed for hormone levels that could foretell a birth.

Snyder said Lun Lun cooperated with an ultrasound Tuesday, but the test was inconclusive. Lun Lun, born in 1997, "has had a pseudopregnancy each year since she was 31/2," Snyder said. "We get excited every year, but you never know for sure."

The four U.S. zoos with pandas have them on long-term loans from China in exchange for payments that fund conservation programs to save the animal in the wild. The pandas at the Memphis Zoo are too young to breed.

The cubs are the property of China and are to be sent there when they are 2 years old. The U.S. Fish and Wildlife Service, which regulates panda imports and care, does so on the condition that panda cubs be included in behavioral, nutrition or other research to improve the animal's status in the wild.

"It's too easy to focus on the good news of a new birth and characterize the new cub as a source of hope for the future of the species," said Karen Baragona, acting director of the World Wildlife Fund's species conservation program.

"The cub is an ambassador, but it's not going to bolster the wild population -- but the zoo, through its contribution, is going to be able to bolster the wild population and protect its habitat."

Bai Yun cuddles with her cub, Mei Sheng, at the San Diego Zoo in May 2004. Zookeepers say Bai Yun is showing signs of being pregnant again.Lun Lun, right, shown in 1999 with Yang Yang at Zoo Atlanta, was artificially inseminated in March. She has been eating less and resting more -- actions that indicate pregnancy.The National Zoo's Mei Xiang cares for her newborn, outlined in the white box. The zoo hopes to be able to examine the cub soon.Tian Tian, left, and Mei Xiang are shown enjoying bamboo at the National Zoo in 2002. Mei Xiang gave birth July 9.