WASHINGTON » A female giant panda at the Smithsonian’s National Zoo is a mother for the second time, giving birth to a cub.

The birth late Sunday by Mei Xiang was a surprise after years of failed pregnancies.

Mei Xiang gave birth at 10:46 p.m., the zoo said. The staff can hear the cub making a squawking noise, but has yet to see it because Mei Xiang has built a large nest in her den. Panda cubs are pink and nearly hairless and about the size of a stick of butter.

"I’m cautiously optimistic as we haven’t seen the cub yet, but we know that Mei is a good mother. Like everyone else, I’m glued to the panda cam for my first glimpse of the cub!" Dennis Kelly, the zoo’s director, said in a news release.

The cub is the second born to Mei Xiang and Tian Tian as the result of artificial insemination. Mei Xiang has had five consecutive pseudopregnancies since 2007 and had a less than a 10 percent chance of being pregnant after so many failed attempts. A pseudopregnancy occurs when a panda ovulates but doesn’t conceive.

Scientists at the zoo had all but given up on Mei Xiang’s chances of conceiving. They worried she had become infertile and had been considering whether to replace her or Tian Tian with other pandas.

Veterinarians will perform the first physical exam after Mei Xiang and the cub have had time to bond, the zoo said. The new mother will most likely not come out of her den, or eat or drink, for at least a week.

Mei Xiang’s first cub, Tai Shan, was born in 2005 and gained a near-rockstar following before being returned to China in 2010. Under the Smithsonian’s panda loan agreement, any cub born at the zoo must return to China for breeding.

Mei Xiang and Tian Tian are the second panda couple to live at the national zoo. The first panda pair, Ling-Ling and Hsing-Hsing, arrived in 1972 as a gift to the American people after President Richard Nixon’s historic visit to China. Ling-Ling and Hsing-Hsing lived more than 20 years at the zoo and had five cubs, but none of them survived.