Lun Lun, the giant panda at Zoo Atlanta, has given birth to twins.

The twins are her fourth and fifth offspring. She was in labor barely 90 minutes, compared to 36 hours for her first delivery.

Both infants appeared healthy, according to Dwight Lawson, deputy director.

Though giant pandas bear twins in the wild, such an event has not happened among the pandas born in the U.S. in the last 20 years, Lawson said.

Lawson said that in the wild, usually only one of a pair of twins will survive because of the difficulty of caring for two.

The zoo hopes to ensure the survival of both infants by switching them regularly, giving them to the mother one at a time.

“It’s tricky,” said Lawson.

Mei Lan, Lun Lun’s firstborn, is in China at the Chengdu Research Base of Giant Panda Breeding. His younger brothers, Xi Lan and Po, are still at Zoo Atlanta.