Updated

A panda at Washington's National Zoo gave birth to a stillborn cub Saturday evening, one day after giving birth to an apparently healthy cub.

Zoo spokeswoman Pamela Baker-Masson said that the panda, Mei Xiang, gave birth to the stillborn cup at 7:29 p.m. local time and spent 17 minutes grooming it before letting it fall to the floor of the panda's den. When zoo officials were able to examine the cub, they found that it was never alive and had developmental abnormalities. A necropsy was underway to determine more information.

Baker-Masson added that Mei Xiang's live cub, born Friday evening, was still doing well, though the mother has not yet allowed zoo staff to get a good look at it so far.

Zookeepers made two attempts at examining the cub Saturday, but Mei Xiang was cradling the cub and officials were unable to take it for a closer examination, Baker-Masson told the Associated Press. They planned to try again Sunday.

When caretakers do get to check out the live cub, which is the size of a stick of butter, they will try to listen to its heart and lungs, record its weight and collect a DNA sample.

Brandie Smith, the zoo's curator of mammals, told the Associated Press she and others are "cautiously optimistic" about that cub's health. She compared the planned exam to a race car pit stop, a fast and highly choreographed checkup before reuniting mom and cub.

The live cub was the 15-year-old panda's third. Mei Xiang gave birth to a cub last year that died after just six days. Its lungs hadn't fully developed and likely weren't sending enough oxygen to its liver. Mei Xiang's first cub, a male named Tai Shan, was born in 2005.

An early exam at the zoo is a change from last year, and staff members made several other changes in preparation for another cub. Mei Xiang's den was altered to allow keepers to get closer to her, and the zoo invited a panda expert from China who specializes in newborns to help out. Two of the zoo's panda keepers also recently spent time in China learning more about examining newborns.

Information collected during the exam will serve as a baseline for future exams. And the DNA sample, either from a swab of the cub's mouth or feces, will be used to determine the cub's father. Mei Xiang was artificially inseminated both with sperm from the zoo's male panda, Tian Tian, and sperm from a panda at the San Diego Zoo, Gao Gao.

Visitors to the zoo Saturday said they were excited about another panda cub. Melissa Schmechel, of Alexandria, Va., said she spent about 30 minutes Friday watching the zoo's online panda camera after it was announced on Facebook and Twitter that Mei Xiang had gone into labor. She said she and her family had made plans to visit the zoo last year after the birth of Mei Xiang's second cub and were sad when it died.

"Hopefully this will have a better outcome," she said as her 11-year-old daughter, Laura, hugged a newly purchased stuffed panda.

The Associated Press contributed to this report.