Mei Xiang, the female giant panda at the Smithsonian's National Zoo in Washington, eats breakfast in this file photo. (Susan Walsh/AP)

The National Zoo’s adult female giant panda has entered a critical phase in her months-long reproductive cycle, the zoo said Monday, suggesting that the cycle may conclude in a few weeks.

The zoo said it doesn’t know if Mei Xiang is pregnant, because female pandas exhibit signs of pregnancy even when they are not pregnant. But zoo experts have detected the rise in Mei Xiang’s hormone levels that marks the final part of her cycle.

Her hormone levels should rise for the next few weeks, and then come down, at which point she will either deliver a cub or have had a false pregnancy, the zoo said.

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 panda in Wolong, 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.

Seeking mates for Mei Xiang and Tian Tian

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.

Panda experts have conducted several ultrasound scans on Mei Xiang this summer and will continue to do so, but the tests have revealed little.

The only way to determine conclusively whether a giant panda is pregnant is to see a fetus on an ultrasound, the zoo said. A giant panda fetus does not start developing until the final weeks of gestation.

The slow rise in hormones started July 20, the zoo said, and the process should conclude in 30 to 50 days.

Mei Xiang has also begun nest-building, spending more time in her den and sleeping more, the zoo said.

The area of the giant panda habitat closest to her den will soon close, because she shows extra sensitivity to noise during the final weeks of the cycle.

Mei Xiang’s popular female cub, Bao Bao, turns 2 on Aug. 23. Bao Bao and her brother Tai Shan, who was born at the zoo in 2005 and is now in a breeding program in China, were sired by Tian Tian.

Unlike in humans, where the egg is fertilized and the fetus starts to grow and can be measured, it isn’t so obvious with a panda, zoo officials said.

A female panda can have what is known as “delayed implantation.” She can have a fertilized egg float in her uterus for weeks before it attaches to the wall of her uterus, according to zoo officials.

In addition, a female panda’s hormones and behavior are the same whether she’s pregnant or not. If she does not give birth at the end of the cycle, she has experienced what is a called a false pregnancy or pseudopregnancy.

Mei Xiang had five consecutive false pregnancies between 2007 and 2012. That year, she became pregnant and successfully gave birth, but the week-old cub died due to liver abnormalities and fluid in the abdomen.