Veterinarians at the Memphis Zoo said they are concerned about the pregnancy of giant panda Ya Ya after an ultrasound failed to detect a heartbeat.

Blood tests have shown the hormone progesterone, which indicates a pregnancy, but at a lower level than it should be, said Matt Thompson, curator of mammals.

"The lower hormones are not what we expected, so this might not be a good pregnancy," Thompson said.

Zoo officials gathered Wednesday to watch radiologist Dr. George Flinn perform the ultrasound to look for a fetus that is believed to be no larger than a ping pong ball.

Ya Ya was artificially inseminated in January after she and a male companion failed at mating the old-fashioned way. Since then, the zoo has closely monitored her as the potential due date draws near for the 6-year-old panda.

She is in her maternity den and has access to a day room where zoo visitors can see her.

Each week, zoo workers have entered a cage to perform an ultrasound, with Ya Ya laying back for treats like banana chips while Flinn rubs a shaved spot on her belly with the ultrasound wand.

Although no heartbeat has been detected, Flinn has seen a gestational sack, where the fetus would develop, and a fetal pole, another sign of a developing fetus.

Ya Ya has been showing behavioral signs of pregnancy, such as occasional agitation, sleeping more often and becoming more secluded.

Panda pregnancies cannot be confirmed until shortly before the delivery date. Gestation lasts about 133 days.

An ultrasound scheduled for next Wednesday is expected to show better results if the fetus is still growing. If the pregnancy is not successful, the zoo plans to try artificially inseminating Ya Ya again.

Pandas are notoriously poor breeders — one reason their species is endangered — and females have only three days a year in which they can conceive.

Only three other U.S. zoos — those in San Diego, Washington and Atlanta — have giant pandas.

On Thursday, officials at Washington's National Zoo said its panda mother might be pregnant for the second time in two years. They cited a spike in Mei Xiang's hormone levels on Wednesday, two months after she was artificially inseminated.

The most recent panda cub born in the U.S. was Mei Lan in Atlanta last September.