This could be the zoo's last chance to breed Mei Xiang. (Photo courtesy of National Zoological Park/Jessie Cohen)

WASHINGTON - The National Zoo's female giant panda isn't pregnant, at least not yet.

Mei Xiang had her second ultrasound examination early Monday since she was artificially inseminated in January. Zoo curators and veterinarians looked closely at the monitor, but there was no sign of a fetus.

Zoo scientists say it could take weeks potentially for a fetus to be visible. Still, they are monitoring to establish a set of data for the future.

This could be the zoo's last chance to breed Mei Xiang and 12-year-old male Tian Tian. A 10-year loan for the pair expires at the end of the year. The bears could be returned to China at that time.

The zoo has inseminated Mei Xiang numerous times, but she has only had one cub, Tai Shan in 2005. He was sent to China last month.

Information from The Washington Post