PANDA WATCH: Ultrasound detects fetus for National Zoo's giant panda Mei Xiang

Veterinarians for the National Zoo believe a fetus has been found in giant panda Mei Xiang after an ultrasound was performed Wednesday morning.

The National Zoo said the fetus is about four centimeters and it is estimated that the female panda could give birth next week or in early September.

"Today, we are cautiously optimistic," said Dennis Kelly, director of the Smithsonian's National Zoo. "We want a healthy cub for all the right conservation reasons. I am excited, but I have to say that we were prepared for a cub even before this morning's ultrasound. 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."

Although optimistic, the zoo warns that it is possible that Mei Xiang could resorb or miscarry the fetus.

Scientists artificially inseminated Mei Xiang on April 26 and 27. In July, the zoo said her hormone levels were rising and she began exhibiting behaviors consistent with pregnancy or a pseudopregnancy. Mei Xiang has had five pseudo-pregnancies between 2007 and 2012.

The National Zoo has closed the panda habitat completely to provide quiet for Mei Xiang, but the zoo's other two pandas, Tian Tian and Bao Bao, can still be seen in their outdoor enclosures.

All three pandas can still be seen on the zoo's panda cams.