updated 12/14/2012 11:50:50 AM ET 2012-12-14T16:50:50

Two Andean bear cubs were born overnight at the Smithsonian's National Zoo, officials at the institution announced.

Six-year-old bear mom Billie Jean gave birth to the twins — one around 12:01 a.m. local time and the other around 2:02 a.m. Thursday (Dec. 13), according to officials at the zoo in Washington, D.C.

Caretakers are monitoring the mom and newborns from the bear cam for now, allowing the three time to bond without interference. Zoo officials said they won't know the cubs' sex for at least two months. The two cubs' father, Nikki, had to be euthanized in August after his health declined; he had been diagnosed with squamous cell carcinoma, or cancer.

This is Billie Jean's second set of cubs. Zookeepers began to suspect this pregnancy in November, and ultrasounds showed two amniotic sacs and then, with later ultrasounds, fetal growth and development. Her telltale "I'm pregnant" signs included a decreased appetite, a nest she built inside a den, and her lack of interest in going outdoors, according to the Smithsonian.

Andean bears (also called spectacled bears ) live in South America and are listed as vulnerable by the International Union for Conservation of Nature (IUCN). The zoo noted that the species has a high infant mortality rate — up to 44 percent in the first year.

"The Andean bear population in human care has experienced a lull in the past six years and these cubs are the only surviving Andean cubs in a North American Zoo since Billie Jean's first cubs, Bernardo and Chaska, were born in 2010," zoo officials said in a statement.

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