It's doubtful the first giant panda cub born at Zoo Atlanta will have a twin, a zoo official said Thursday.

Veterinarians had been watching Lun Lun, a 9-year-old giant panda, in case she gave birth to another cub.

Lun Lun gave birth Wednesday after going into labor for a record 35 hours, and there was a 50 percent chance that she'd give birth to another cub within a day.

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• Click here to read about the sad fate of a Chinese newborn panda.

But scientists determined that the panda's chances to give birth again were "very, very slim," Thursday night, said Susan Elliott, the zoo's spokeswoman.

"We're going to continue watching through the night, but we haven't seen any behaviors that would indicate she'd be in labor again," she said.

The cub was the fifth giant panda born at a U.S. zoo in the last six years. Only about 75 births have ever been recorded.

The zoo had tried to impregnate Lun Lun for seven years. She was successfully artificially inseminated with semen taken from her male partner, Yang Yang.