Mei Xiang was inseminated twice in late January with sperm from Tian Tian. (Matt McClain/FOR THE WASHINGTON POST)

For the fifth year in a row, the National Zoo announced Friday that despite its intensive scientific efforts, its female giant panda, Mei Xiang, has again failed to get pregnant.

After days of waiting for the panda’s months-long reproduction cycle to finish its course, and without the appearance of cub, disappointed zoo officials concluded that Mei experienced a false pregnancy.

It was her sixth false pregnancy in total.

The announcement means that the zoo probably will try only one more time for a panda pregnancy with Mei. If she does not become pregnant during next year’s cycle, she, and perhaps her mate, Tian Tian, are liable to be replaced, zoo officials have said.

The zoo is determined to produce panda cubs in Washington, with or without the current pair.

As in past years, there had been some optimism as the Zoo entered the height of its annual panda pregnancy watch June 20. Volunteers began manning the zoo’s panda cams 24 hours a day to see if Mei might give birth.

Mei’s reproduction cycle ran late this year, and toward the end she was alternately exhibiting signs that she was and was not pregnant. Last year, the zoo discovered she was not pregnant in late April. The year before, it was late May.

Mei has been artificially inseminated each year since the year after her only cub, Tai Shan, was born in 2005. Tai Shan was sent to a breeding program in China last year.

Mei experienced one false pregnancy before Tai Shan was born, the zoo said.

This year Mei went into heat in January. She and Tian Tian had undergone a special exercise regimen to see if it would help them mate naturally. Alas, they tried, but failed, the zoo said.

So Mei was inseminated twice in late January with sperm from Tian Tian.

The panda gestation period usually lasts 90 to 185 days.

But giant panda reproduction is mysterious. Female pandas can exhibit many signs of pregnancy without being pregnant. And experts often must simply wait until the panda’s cycle ends — with or without a cub.

Mei in recent weeks had been spending more time in her den and was eating very little, the zoo said last month. Both could have been signs that she was pregnant, or undergoing what is called a false or pseudopregnancy.

The zoo had been attempting to perform ultrasounds several times a week since early June. But Mei was “picking and choosing which times are agreeable to her,” the zoo said in a recent posting on its Web site.

On June 30 Mei resumed eating and became more active, both signs that she might not be pregnant. An ultrasound performed July 1 showed no evidence of a fetus. But then she became inactive again, and resumed “nesting” behaviors, puzzling zoo scientists.

Last January, the zoo announced a new agreement with China that extended the potential stay of the two giant pandas for five more years.

Giant pandas are native to China, and the Chinese own and lease all giant pandas in U.S. zoos.

Government and zoo officials had been trying to get one or two new pandas, because in 10 years, Tian Tian and Mei Xiang have produced just one cub.

Dennis W. Kelly, the zoo’s director, had made it clear that he would like to replace one or both of the giant pandas to enhance the likelihood of having more cubs at the zoo.

The new agreement replaced a 10-year lease that expired last Dec. 6.

Among other things, it lowered the annual cost of leasing the pandas from $1 million to about $500,000. The money is to go toward panda conservation programs in China.

It called for an intensive, China-U.S. study of the pandas this year and next year to try to determine why they have produced only one cub.

And it left the door open to the possibility of replacement pandas if the study concludes that one or both animals are unsuitable for breeding, officials said.

Zoos in Atlanta and San Diego have had more success breeding pandas, producing more than six cubs in recent years.

The agreement also allows any panda cub born at the zoo to stay for four years.

The zoo’s pandas — both born at the China Research and Conservation Center for the Giant Panda in Wolong, Sichuan Province — arrived in Washington on Dec. 6, 2000.

Tai Shan, was born via artificial insemination on July 9, 2005.

Recent science suggests that if a female giant panda has not become pregnant for several years, it is increasingly unlikely that she will be able to do so.

“We’ve not had a cub here in the last five years,” Kelly said in an interview last summer. “What we are talking about is, what can we do to ensure that there is a cub in our future?”