Washington - Ultrasound tests on Mei Xiang, the female giant panda at the Smithsonian National Zoo in Washington, indicate she might be pregnant, officials said on Wednesday.Veterinarians detected what they think is a feotus which, at 4cm suggests Mei Xiang could give birth next week or in early September.It's possible that the panda, the mother of two surviving cubs could resorb or miscarry the fetus, but zoo officials remained hopeful."Today, we are cautiously optimistic," said National Zoo director Dennis Kelly in a statement that included an image from the ultrasound."We want a healthy cub for all the right conservation reasons," he said."Our expert team of keepers, scientists and veterinarians are going to do exactly what they are trained to do, and I'll just ask everyone to remain positive with us."Mei Xiang was artificially inseminated in April with frozen semen from a male giant panda named Hui Hui that resides at the China Conservation and Research Centre for the Giant Panda in Sichuan province.Mei Xiang's ultrasound. (Smithsonian National Zoo, AFP)She was also inseminated with fresh semen from the zoo's male giant panda Tian Tian.If Mei Xiang gives birth, DNA tests will establish which is the father.Mei Xiang exhibited signs of pregnancy in July that included sleeping more, eating less, building a nest and spending more time in her den.But on Wednesday marked the first time that she responded to veterinarians' calls to approach them for an ultrasound scan of her uterus.Besides Mei Xiang and Tian Tian, the National Zoo is home to their female cub Bao Bao, who turns two on Sunday. Her brother Tai Shan, born in 2005, is in China.On Tuesday, Malaysia announced that a giant panda at its National Zoo, Liang Liang, had given birth. The newborn's sex has yet to be determined.Last month, another giant panda, Jia Jia, became the oldest-ever giant panda in captivity when she turned 37, the human equivalent of more than 100 years at a Hong Kong theme park.There are fewer than 2 000 pandas now left in the wild, according to the World Wildlife Fund, as their habitats have been ravaged by development.Roads and railways cut through the bamboo forests they depend upon in China's Yangtze Basin, their primary habitat.Pandas rely on bamboo and eat almost nothing else. Given their low birthrate, captive breeding programs are key to ensuring their survival.