Mei Xiang, the National Zoo's giant panda, gave birth to a panda cub Sunday night — her second — after five false alarms in a row.
The new cub was born at 10:46 p.m. Sunday. The zoo announced the news about four hours later.
Zoo officials said they believed only one cub had been born, but they should know by early Monday if the cub has a twin.
"Mei Xiang is behaving exactly the same way she did when Tai Shan was born," chief veterinarian Suzan Murray said in a statement.
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"She is cradling her cub closely, and she looks so tired, but every time she tries to lay down, the cub squawks and she sits right up and cradles the cub more closely. She is the poster child for a perfect panda mom," she said.
Mei was artificially inseminated twice in April — including once while zoo staff live-tweeted.
Vets used sperm collected and frozen from Tian Tian in 2005. That might have been a particularly good year for him, as 2005 was also the year that brought us Mei Xiang and Tian Tian's first cub, Tai Shan.
"One factor that may have helped Mei Xiang conceive a cub was her return to a more normal estrus cycle," the zoo said.
Mei went into estrus this year in late April. From 2009 to 2011, she went into estrus in January. Tai Shan was born in a year when Mei began her cycle early.Video: Panda birth celebrated at San Diego Zoo
The odds of Mei conceiving a cub were less than 10 percent, the release said, after five 'pseudopregnancies' in which she showed all the signs of expecting — but without the cub.
Zoo vets had not detected a pregnancy via ultrasound leading up to the cub's birth, but they hadn't for Mei Xiang and Tian Tian's first cub, either.
Tai Shan — dubbed Butterstick by D.C. panda fans, since he was the size of a stick of butter at birth — was born July 9, 2005. He moved to China in 2010 to join a breeding program.
According to the zoo, keepers began to prepare for a birth again after they saw Mei "spending extended periods of time" licking and cradling toys.Video: Panda census a bear of a task
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As happened with the birth of Tai Shan, all our enjoyment of the new cub will be via the grainy panda cam for the time being.
Vets will not be able to examine the cub for a few weeks, to give Mei and the baby time to bond. They said Mei most likely will not come out of her den, eat or drink for at least a week.
An average panda pregnancy lasts for a range of between 90 to 160 days. The average is 135 days, which had provided a rough due date of Sept. 10-12.
NBCNews.com staff contributed to this report.