Mei Xiang at the National Zoo. (Courtesy of the National Zoo)

The National Zoo said Wednesday that experts believe they have spotted a developing fetus inside its female giant panda Mei Xiang.

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 .

The fetus was spotted by chief zoo veterinarian Don Neiffer during a morning ultrasound test.

He said he spotted what appeared to be a rib cage and some cranial structure, and saw the fetus move.

“Everything points to this being a really healthy, normal pregnancy at this point,” he said Wednesday. “I am expecting this to progress nicely.”

National Zoo veterinarians believe they have detected a developing fetus during an ultrasound procedure on giant panda Mei Xiang. (National Zoo)

He said the panda could give birth as early as seven to 10 days, or as late as 21 to 28 days. He said it looked like it might be sooner rather than later.

Neiffer cautioned, however, that the fetus could also be “resorbed,” which can happen in bears, and a birth might not occur. “The ability for this fetus to become unviable and be resorbed from now until the time when we would expect delivery is a real possibility,” he said.

“Don’t know what the chances of that are, but it is a possibility,” he said.

He said experts have been trying to conduct regular ultrasound tests, which give an image of the uterus, on the panda. But Mei Xiang is generally reluctant to participate in the tests as her pregnancy cycles conclude, which is the time a fetus might be seen in a giant panda.

Neiffer said keepers alerted him Wednesday morning, however, that Mei looked like she might cooperate. Neiffer said he took his ultrasound equipment to the panda complex.

He tried three or four times to conduct the test, but each time Mei Xiang walked away. “We thought that we weren’t going to be successful,” he said.

“On the last attempt,” she submitted, reclined and was presented with honey water as a reward.

Seeking mates for Mei Xiang and Tian Tian

He ran the ultrasound probe over a shaved area on her abdomen, “and in a very short period of time was able to identify . . . fetal tissue in an embryonic sack in her uterus,” he said by telephone from the zoo.

“We were able to take some images as well as some short video clips,” he said. “I was not able to get a fetal heart rate. We did get a nice, healthy heart rate from mom. And at one point did actually see fetal movement.”

“All signs point to us currently having a healthy, viable fetus, based on the ultrasound,” he said. “So we are cautiously optimistic.”

He said Mei Xiang’s heart rate was a healthy 40 beats per minute. “That supports that the uterine structures are getting a nice, healthy blood supply,” he said.

“I was really hopeful that I would be able to see the heart and get a fetal heart rate,” he said. “One, because it’s cool. And two because it obviously confirms viability. But Mom was done with it at that point. So we took what we could get.”

The zoo said it will now be on 24-hour-a-day panda pregnancy watch, where staff and volunteers monitor Mei Xiang to see whether she gives birth.

The giant panda was artificially inseminated twice — on April 26 and 27 — with semen collected from the zoo’s male giant panda, Tian Tian, and from a male giant panda at a research center in Wolong, China.

The Wolong panda, Hui Hui, was picked because he was a good genetic match for Mei Xiang, the zoo said. The sample from Hui Hui was frozen and flown from China to the zoo’s cryopreservation bank.

If Mei Xiang has a cub, a DNA test can be used to see which male sired the cub. The gestation period for a female giant panda is between three and six months.

Mei Xiang began nest-building, spending more time in her den and sleeping more recently, the zoo said.

Mei Xiang has given birth to two surviving cubs: Tai Shan and Bao Bao. Tai Shan was born July 9, 2005, and he now lives in China. Bao Bao was born Aug. 23, 2013

This story has been updated.