After weeks of anticipation, Zoo Atlanta’s 15-year-old giant panda Lun Lun gave birth to twins on July 15.

The birth of the second cub was a surprise to zoo staff, who didn’t know she’d been pregnant with twins. Although it’s not unusual for giant pandas to bear twins in the wild, Lun Lun’s cubs are the first giant panda twins born in the U.S. in 20 years.

giant panda Lun LunLun Lun, who was artificially inseminated in March, was in labor for 90 minutes before giving birth to her fourth and fifth cubs, which both appear healthy.

In the wild, usually only one twin survives because it’s difficult to care for two cubs, but zookeepers hope to ensure the survival of both infants by giving them to Lun Lun one at a time, switching the cubs out every few hours.

Newborn giant pandas are tiny, and there is a high risk of mortality in the first few months. This risk increases in twins, which tend to have lower birth weights than single cubs. The combined weight of Lun Lun’s cubs is 8.6 ounces.

“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,” Zoo Atlanta President and CEO Raymond B. King said in a news release.

The sex of the newborn cubs hasn’t been revealed yet, and the tiny pandas won’t make their public debut at the zoo until late fall.

The twins’ father, 15-year-old Yang Yang, and older brothers, Xi Lan, 4, and Po, 2, remain on exhibit and will not be housed with Lun Lun or the cubs. Giant pandas are solitary in the wild, so this separation is normal.

Mei Lan, Lun Lun’s firstborn, lives at the Chengdu Research Base of Giant Panda Breeding in China.

All five of Lun Lun’s and Yang Yang’s cubs were conceived through artificial insemination.

You can follow the twins’ progress on Zoo Atlanta’s PandaCam, as well as on on FacebookTwitterGoogle+ and Instagram. On the PandaCam page, you can also subscribe to Cub Confidential for exclusive email content about the newborns.

Giant pandas are the rarest members of the bear family and are listed as endangered, with fewer than 1,600 believed to remain in the wild.

For more information on Lun Lun's cubs, watch the video below.

Photo: (Lun Lun) Erik S. Lesser/Getty Images

More panda stories on MNN:

Laura Moss writes about a variety of topics with a focus on animals, science, language and culture. But she mostly writes about cats.

It's twins for Atlanta's giant panda
The cubs are the first giant panda twins born in the U.S. in 20 years.