WASHINGTON, July 6 (Reuters) - Despite efforts to breed another giant panda cub, the female at the U.S. National Zoo on loan from China, Mei Xiang, is not pregnant after all, officials said on Friday.

The black-and-white giant panda is one of the world's best-known endangered species and breeding is difficult.

Mei Xiang had been artificially inseminated in early April but zoo scientists said they have not seen a fetus during ultrasound exams as a hormone associated with pregnancy began to decline in recent days.

"Female giant pandas almost always undergo a pseudopregnancy when they ovulate but fail to conceive," the zoo said in a statement.

Mei Xiang experienced hormonal changes and behaviors like those of a true pregnancy, as she has done three times previously. In 2005, she gave birth to one cub, Tai Shan.

Giant pandas typically ovulate once a year and the zoo said it would consider next year whether to try again.

As part of the original 10-year deal to bring Mei Xiang and male panda Tian Tian to the United States, any offspring were to be returned to China after the cub's second birthday. However, in April Beijing agreed to allow the cub to stay for another two years.

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