Wed Aug 3, 2:09 PM ET
SAN DIEGO - A 13-year-old giant panda gave birth to a cub at San Diego Zoo, but a second fetus died in the womb, officials said Wednesday.
Bai Yun gave birth shortly before 10 p.m. Tuesday after being in labor for three hours. The birth  less than a month after another cub was born at the National Zoo in Washington  was captured on closed-circuit television in the Giant Panda Research Station birthing den.
However, hope of a third panda birth was dashed as Zoo Atlanta announced that tests showed its panda was not pregnant. There are only about 1,600 giant pandas in captivity and in the wild, zoo officials said.
San Diego Zoo officials said the cub weighed 4 ounces, the size of a stick of butter. The gender will not be known for some time because zoo officials don't want to disrupt mother and child.
"The birth of a giant panda cub is definitely something to celebrate," said Yadira Galindo, the zoo's spokeswoman.
Breeding pandas in captivity is a particularly difficult process, in part because females are in heat for only a day or two a year, and many cubs do not survive infancy. It wasn't until 1999, when Bai Yun had a female cub, Hua Mei, that a panda born in a U.S. survived into adulthood. Bai Yun successfully gave birth to a second cub, a male, in 2003.
Two fetuses were detected last month during a routine check of Bai Yun, who had mated with a male in April. However, on Monday the zoo's veterinary staff said one of the fetuses had died in the womb. Experts said it appears to be common for giant pandas to conceive twins and then lose one fetus; the same thing happened during Bai Yun's 2003 pregnancy.
The panda born Tuesday joins a cub born July 9 at the National Zoo in Washington. Zoo officials there announced Tuesday that they had finally been able to examine the little creature while the panda's mother, 6-year-old Mei Xiang, left the den to eat, and determined it is male.
Last month, Zoo Atlanta officials had said their 7-year-old giant panda, Lun Lun, was showing symptoms of a possible pregnancy. She had been artificially inseminated in March. But spokeswoman Jacqueline Petty said Wednesday that experts had determined this week that it was a false pregnancy.
"We would have been ecstatic" if Lun Lun were pregnant, Petty said. The zoo had already set up a nursery and built a special bed for the giant panda to give birth on, she said.
Lun Lun can't try for cubs again until mating season next spring, Petty said.
However, Zoo Atlanta officials were thrilled by the births in San Diego and Washington. "A birth somewhere else is like a birth here," said Petty.
Gestation in giant pandas has been estimated at 97 to 163 days, making it difficult to predict a birth date accurately.
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On the Net:
San Diego Zoo: http://www.sandiegozoo.org/zoo/ex_panda_station.html
National Zoo: http://nationalzoo.si.edu/Animals/GiantPandas
Zoo Atlanta: http://www.zooatlanta.com/animals(underscore)giant(underscore)panda.htm
File photo from 2001 of Hua Mei, a baby giant panda born at the San Diego Zoo on August 21, 1999.
A new giant panda cub has been born at the San Diego Zoo. Bai Yun, a 13-year-old giant panda, gave birth at 9:57 p.m. on Tuesday to a cub weighing four ounces (113.5 grams).

"Bai Yun immediately tended to her squawking infant," said Suzanne Hall, a panda research technician at the San Diego Zoo who observed the births of Bai Yun’s other cubs in 1999 and 2003. The cub is the second to be born in the United States this year. In Washington, Mei Xiang gave birth to a male panda cub at the National Zoo last month after artificial insemination.

According to Hall, early ultrasound images indicated Bai Yun was pregnant with twins. However, zoo personnel became suspicious that one fetus was no longer viable after veterinarians failed to detect a heartbeat in one cub last week. Bai Yun failed to give birth to a second cub Tuesday evening. "It is likely that the second fetus died in utero and was reabsorbed by her body," Hall said.

Panda behavioral researchers will be watching the pair 24 hours a day for three weeks to record and assess the mother-infant relationship. The new cub has not yet been named. The gender of the cub will be unknown until zoo staff can safely check.