Female panda Mei Xiang snacks on bamboo in 2001. (By Marvin Joseph/The Washington Post)

That could mean she’s “with cub” or it could mean, as it so often has for pandas in captivity, that she has a false pregnancy.

Hormone levels aren’t enough to know if Mei Xiang (may-SHONG) is actually pregnant. Zoo veterinarians have been conducting ultrasounds twice weekly, but they have not detected a fetus.

Panda fetuses don’t start developing until the final weeks of the 160-day gestation period.

That’s only one reason pandas find it so hard to have offspring. When scientists step in to help things along, they only have about 48 hours once a year, when the female is in heat, to try to achieve pregnancy.

A Chinese panda breeding expert helped zoo scientists artificially inseminate Mei Xiang in January. She and male panda Tian Tian (tee-YEN tee-YEN) had attempted to mate but weren’t successful.

Their only cub, Tai Shan (tie-SHON), was born July 9, 2005.