FILE - Mei Xiang, the female giant panda at the Smithsonian's National Zoo. (Susan Walsh/AP)

The National Zoo’s annual giant panda pregnancy vigil has moved into its round-the-clock watch phase, zoo officials said Thursday.

Since Aug. 7, volunteers have been monitoring live panda cams all day and night, as the end of the giant panda’s gestation period approaches.

The zoo closed down the panda house Aug. 8 after announcing July 26 that Mei Xiang was experiencing the usual secondary hormone rise that signals the closing of the pregnancy period.

It is difficult to determine if a giant panda is pregnant because the animal goes through the same physiological stages whether it is bearing a cub or not.

“Keepers say that Mei’s behavior is still very lethargic,” the zoo said in a statement. “She is spending more time in her den, sleeping and shredding bamboo for her nest, and eating less.

“She has also occasionally been cradling and grooming her kong toy and boomer ball while in her den. Our veterinarians are still attempting daily ultrasounds . . . but she has been choosing not to participate. All of these behavioral changes are consistent with a pseudopregnancy or pregnancy, and are behaviors that we have seen every year during this time.”

The conclusive evidence is either the arrival of a cub or the close of the period with no cub.

Mei Xiang gave birth in September to a four-ounce female cub, but it died six days later. The cub had underdeveloped lungs that gave rise to liver problems.

Mei Xiang had five consecutive false pregnancies between 2007 and 2012.

This year, officials said, Mei Xiang was artificially inseminated twice March 30, after natural breeding attempts with the zoo’s resident male panda, Tian Tian, were unsuccessful.

During the first procedure, she was inseminated with fresh sperm from Tian Tian and samples collected in 2003 that were frozen.

In the second procedure, Mei Xiang was inseminated with Tian Tian’s frozen 2003 sperm, as well as frozen sperm from the San Diego Zoo’s male giant panda, Gao Gao.

The zoo monitors Mei Xiang’s hormones and behavior for clues to the stages of gestation.

Mei Xiang has started to build a nest, which is consistent with the hormonal changes she is experiencing.

Zoo officials think that her pregnancy period will close around the end of August.