A 13-year-old giant panda gave birth to a cub at the San Diego Zoo (search), but a second fetus died in the womb, officials said Wednesday.

Bai Yun (search) 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 (search) 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 the 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.