An ultrasound image of the National Zoo's giant panda Mei Xiang is pictured in this handout photograph taken and released on August 19, 2015. [Photo/Agencies]

WASHINGTON - The Smithsonian National Zoological Park said Wednesday that its adult female panda was carrying a developing fetus and could give birth in coming weeks.

On Wednesday morning, veterinarians detected a developing fetus during an ultrasound procedure for Mei Xiang, one of the two adult giant pandas which arrived here from China on Dec 6, 2000, the zoo said in a statement.

"Based on the size of the fetus, which is about four centimeters, veterinarians estimate that Mei Xiang could give birth early next week, or possibly in early September," said the statement.

However, the zoo cautioned that "substantial possibility" exists that Mei Xiang could resorb or miscarry a fetus.

"Today, we are cautiously optimistic," said Dennis Kelly, director of the zoo.

Since her introduction to the National Zoo, Mei Xiang has given birth to two surviving cubs, Tai Shan and Bao Bao. Tai Shan, born on July 9, 2005, now lives in China, and Bao Bao, born on Aug 23, 2013, will return to China after turning four.

Giant panda Mei Xiang plays inside the Snithsonian's National Zoo in Washington, in this handout image taken on April 19, 2015 and obtained on August 10, 2015. [Photo/Agencies]