Panda at National Zoo pregnant? Hard to say

Could a baby panda be on the way at the National Zoo in Washington, D.C.? Well, the experts at the zoo aren't really sure!

Why?

CBS News Correspondent and resident "Early Show" veterinarian Dr. Debbye Turner Bell visited the zoo to find out.

On the show Tuesday, Bell explained it's not so simple to tell when a panda is pregnant.

Bell got a very rare, behind-the-scenes look at Mei Xiang, the female giant panda at the National Zoo, getting an ultrasound.

How, exactly, does one give a panda an ultrasound?

Bell said, "First, you have to get the bear to come out of her enclosure. That's the easy part."

Then, zoo veterinarian Dr. Suzan Murray must get Mei Xiang, 13, to lie still for the examination.

A steady stream of apples, pears, and carrots keep Mei Xiang occupied. But Murray still has an uphill battle.

Murray said as she gave the ultrasound, "There's a lot going on in there. There's big loops of bowel filled with bamboo. It's hard to see the reproductive tract - even when you have such a compliant patient."

Pandas are fertile for just 24 hours, once a year. So when one of the four female pandas in U.S. zoos gets pregnant, it's a big deal. But the tricky part is, it's almost impossible to predict the event in advance.

"We know for sure that a panda is pregnant when a little baby is born," Murray said. "Very often, that's the only way."

Each year, a female can have a real pregnancy or a false one, called a pseudo-pregnancy.

Murray explained, "The hormonal profile is identical whether she's pregnant or not pregnant. And then the ultrasounds, up until the last 10 days, are also identical. So it's very difficult to figure that out."

The last time a panda cub was born at the National Zoo was six years ago. The arrival of little Tai Shan caused an international stir.

Smithsonian National Zoo Senior Curator Brandie Smith said, "When Tai Shan came along, the entire country, the entire world was excited about it."

With only 1,600 in the wild, giant pandas are extremely endangered. There are 11 pandas in captivity in U.S. zoos.

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Smith said, "People were watching him on web cam all over the world, trying to get their glimpse of Tai Shan."

But for now, Murray and zoo officials wait and watch Mei Xiang's every move.

Smith said, "This year, her behavior seems to be a lot stronger than in previous years. She's building a huge nest and cradling toys. So this year, we're a little extra hopeful that she may be pregnant."

If there is a little panda bun in the oven, Bell said, Mei Xiang is expected to give birth in the next three weeks.

"It's not just a panda having a baby," Smith said. "It's Mei Xiang. (She) might have her second cub. And we get really excited about it. For us, for her, for everyone."

If Mei Xiang is pregnant, it will be the first panda birth in the U.S. this year. Murray will continue doing ultrasounds three times a week until she sees something, Mei Xiang gives birth, or until nothing happens at all, which, of course, would mean it was a false alarm.