ATLANTA  —  After seven years of trying and hoping, Zoo Atlanta officials announced a rare giant panda birth Wednesday, one of only a few in the United States.

Lun Lun, a 9-year-old giant panda, gave birth just before 5 p.m. after 35 hours of labor. It's the first panda cub born at the Atlanta zoo and is just the fifth giant panda born at a U.S. zoo in the last six years.

The cub is hairless, weighs about 4 ounces and is the size of stick of butter. The gender will not be known for weeks.

• Click here for the Zoo Atlanta Panda Cam.

• Click here to read about twin panda cubs being born in China.

Panda keepers will remain on watch for up to 24 hours to see whether the baby panda has a twin. Fifty percent of panda births result in twin cubs.

Lun Lun is responding normally to her new baby, holding the cub and reacting when it cries, said Dwight Larson, vice president for animal programs and science for the zoo.

The first few weeks of a cub's life are critical to its survival, Larson said.

"These are small offspring and quite fragile," Larson said. "It's going to be tense for us."

Cubs typically take about 75 days to open their eyes. At 100 days, the zoo is planning a naming ceremony for the cub and to present it to the public.

After trying for seven years, the zoo successfully artificially inseminated Lun Lun at the end of March with semen taken from her male partner, Yang Yang. The father and cub are being kept separate, which is normal in the wild.

A panda cub at the National Zoo in Washington, D.C., born last summer, was the product of artificial insemination. There are only 1,600 to 3,000 of the endangered species remaining in the wild today, and another 185 living in captivity.

Only three other U.S. zoos — San Diego, Memphis and the National Zoo — have pandas. Memphis has not been able to successfully impregnate a panda.