The Smithsonian National Zoological Park said Monday that the hormone levels of its adult female panda were rising, a sign that she might be pregnant.

Mei Xiang, one of the two adult giant pandas which arrived here from China on Dec. 6, 2000, has started to show a secondary rise in her urinary progesterone levels since July 20 after she was artificially inseminated on April 26 and 27, the zoo said in a statement.

According to the statement, the increase indicates that she will either have a cub or experience the end of a pseudo-pregnancy within 30 to 50 days.

Noting that it is difficult to determine pregnancy in a female panda, the zoo said the only way to definitively determine if pregnancy exists is by detecting a fetus on an ultrasound. However, giant panda fetuses do not start developing till the final weeks of gestation, said the zoo.

Meanwhile, the zoo said Mei Xiang had begun exhibiting behaviors consistent with a rise in urinary progesterone, such as nest building, choosing to spend more time in her den, sleeping more and eating less.

Since her introduction to the National Zoo, Mei Xiang has given birth to two surviving cubs, Tai Shan and Bao Bao, both born as the result of artificial inseminations.