Four months after the National Zoo artificially inseminated its female giant panda, officials there are launching a "pregnancy watch" today to try to answer a question that has become one of the zoo's favorite diversions:

Is she or isn't she?

Births to giant pandas have been recorded anywhere from 83 to 185 days after mating. It's been 122 days since Mei Xiang, the female, was vaginally inseminated with sperm from Tian Tian, the male, in May. Hormonal analysis indicates that either a birth or the end of a false-alarm period called "pseudo pregnancy" is close at hand.

With little known for certain about the animals' reproduction, zoo officials are adding an extra evening shift of volunteers today to monitor Mei Xiang's behavior, looking for any sign -- nest building, increased licking -- of pregnancy.

But zoo officials have described their efforts as "a long shot." The procedure performed in May is not as effective as depositing sperm directly into the uterus, and they used only about one billion of Tian Tian's sperm, compared with the 10 billion to 15 billion in a natural mating.

"We don't want to get too excited and be disappointed, but then it's hard not to get excited about possibly having a panda cub," said zoo spokeswoman Peper Long. "These bears really have a way of keeping you guessing."

Ever since the giant pandas arrived in December 2000 from China, the mating habits, or lack thereof, of the chunky, black-and-white bears has been the subject of headlines and speculation. "Get some Barry White going on, maybe that'll help," D.C. Mayor Anthony A. Williams (D) joked in May.

Timing is crucial. A female giant panda is in heat once a year for a mere two days. Last year, Mei Xiang, which means "beautiful fragrance," had been rebuffing the advances of her partner until, on April 4, they mated for the first time in a brief encounter that lasted about 15 seconds. No pregnancy resulted.

This year, they attempted to mate repeatedly before hundreds of spectators and 21 zoo cameras. But Tian Tian, which means "more and more," appeared to lose interest, and Mei Xiang's peak window of fertility passed without a successful mating, leading zoo scientists to perform the artificial insemination.

Lacking a fail-safe pregnancy test, zoo specialists are using weekly ultrasound exams, detailed urine analysis and careful observation using a bank of video monitors, recording equipment and computers in the Panda House.

Complicating matters are the "pseudo pregnancies" female giant pandas typically experience, displaying all the signs of being pregnant though they are not. "It's like their bodies practice every year, regardless of whether they've conceived or not," said Lisa Stevens, assistant curator for primates and pandas.

The zoo will run its pregnancy watch until Mei Xiang's progesterone levels decrease sharply, which occurs whether the pregnancy is real or not. Monitoring will then go to 24 hours. Zoo officials estimate that the decrease may occur in mid-September.

Yesterday morning, the zoo's female star lumbered into a steel cage for her ultrasound exam. She appeared unafraid, lounging on her back and exposing her furry tummy as she was hand-fed mouthfuls of apple slices and vitamin-rich, soy-based biscuits. Her partner relaxed in the sun, indifferent to the carefully orchestrated weekly ritual.

Both animals have been trained to cooperate during exams. Tian Tian had some blood drawn yesterday before the ultrasound on Mei Xiang. He sat down in the cage and stuck his paw through a hole to expose his forearm to the needle. Animal keeper Laurie Perry kept him preoccupied with soothing words and a bowl full of apples.

Veterinarian Carlos Sanchez and reproductive physiologist Katey Pelican took turns running the ultrasound's probe over Mei Xiang's abdomen. On the machine's small monitor appeared a murky, black-and-white image of the panda's bladder, colon and part of the uterus. One horn of her Y-shaped uterus appeared to be slightly bigger than before.

But, Pelican said, in what has so far proved a weekly disappointment, "we're not seeing any positive sign of a fetus."

Laurie Perry, left, a keeper in the National Zoo's Panda House, soothes Mei Xiang as veterinarians Carlos Sanchez and Katey Pelican do an ultrasound exam.Sanchez holds a vein in Mei Xiang's paw to stop the bleeding after drawing some of blood for tests. The panda is under a constant pregnancy watch.