Lun Lun, the giant panda living at Zoo Atlanta, gave birth to twins on Monday, shocking her caretakers.

Lun Lun–who has three other cubs–had an ultrasound done last month that showed just one cub, so when she birthed two, it was a surprise to everyone. The new babies are the first giant pandas to be born this year in the U.S.

Mama bear is doing well and zoo officials are thrilled to have two healthy cubs, as having even one survive is a victory.

“We’re thrilled about the possibility of a fourth cub for Lun Lun, but we remain cautiously optimistic at this point,” Zoo Atlanta’s president, Raymond B. King, said last month. “Giant panda cubs are extremely fragile, and the chance remains that the fetus does not go to term. A birth would be another success for our giant panda program, and we share our optimism with our fellow U.S. zoos housing this iconic species and with our colleagues at the Chengdu Research Base of Giant Panda Breeding in China.”

Lun Lun’s caretakers are being careful not to overwhelm her with mothering duties, since it’s not uncommon in the wild for giant pandas to abandon the weaker of two babies. They say they are rotating the cubs so they’ll get care from both their mom and in the zoo’s nursery.

“We’re thrilled to welcome Lun Lun’s and Yang Yang’s twins,” said King. “Twins are an entirely new scenario for Lun Lun, Zoo Atlanta and our animal care teams, who will no doubt be extremely busy over the next few months.”