The round-the-clock pregnancy watch is down to its last few days, but National Zoo keepers still do not know whether their nest-building, apple-cuddling, sleepy female giant panda is about to become a mother.

Yesterday, a hormone test of Mei Xiang's urine kept their hopes alive, but pandas also have false pregnancies that mimic real ones. Scientists had hoped to detect a fetus, which weighs a quarter-pound, by means of a sonogram, but Mei Xiang has refused to sit still for an ultrasound since June 20.

The hormone readings, though, tell them that the waiting should be over -- one way or the other -- by the middle of next week.

"It will be any time now," zoo spokeswoman Peper Long said. "We'll have a cub, or her hormones will go down to normal."

Mei Xiang was artificially inseminated March 11. A daytime pregnancy watch began in mid-June after her hormone levels rose, escalating to the 24-hour watch June 27. Her hormone levels began falling about a week ago, as they should near the end.

The panda will be 7 years old this month -- prime breeding age. Most of the time, she is holed up in a back den at the Panda House, visible to the zoo's pregnancy-watchers only via one of the 23 cameras scattered throughout the indoor-outdoor enclosure. For now, the public can see the panda and her mate, Tian Tian, on the zoo's Web site, but the cameras will go dark for 24 hours after a birth because the Animal Planet cable network bought exclusive rights, Long said.

Mei Xiang has been spending much of her days in a lethargic doze.

Occasionally, she licks herself, and that is when the panda-watch volunteers lean forward eagerly, because if it goes on long enough, it could be the prelude to a birth. The zoo's previous pair of pandas produced five cubs in the 1980s, but none lived more than a few days.

Pandas are notoriously poor breeders. Females come into heat for two or three days a year, false pregnancies are common, and cubs are so small that sometimes their mother accidentally crushes them. This worsens their chances of survival in the wild, where only about 1,600 are left in the shrinking bamboo forests of China.

Mei Xiang and her mate, who arrived in 2000 on a 10-year loan from China, have not had any offspring.

The pair mated briefly in 2003, without result. They attempted to mate last year, after which Mei Xiang was vaginally inseminated, and a false pregnancy followed. The procedure this year injected Tian Tian's sperm directly into her uterus, and zoo officials say it has a 55 percent success rate.

Nancy Schneck of Great Falls, a volunteer at the Panda House, finished a three-hour solo shift Tuesday night, writing down panda behavior observations in response to a timer that goes off every five minutes. She is among 50 Friends of the National Zoo volunteers who are gathering data that FONZ spokesman Matt Olear said may some day help scientists diagnose a pregnancy by monitoring behavior.

Schneck is scheduled to be back today and again tomorrow. Her years of disappointments have not dimmed her expectations. This year, she has noticed that Mei Xiang seems to be licking her thumb a lot. Could she be practicing for taking care of a baby?

"We just hang on to every little straw," she said.

Chief veterinarian Suzan Murray attempts an ultrasound examination Tuesday on Mei Xiang to determine whether the giant panda is pregnant. Zookeeper Laurie Perry tries to distract the panda.