The officials told ABC News a sample of the giant panda's urine exhibited higher levels of the hormone progesterone, an indication that she could be carrying a cub.
"We have now entered a window of 40 to 50 days which will dictate whether a cub will be born," said Brandie Smith, senior curator at the National Zoo. "We have the nursery ready."
The zoo has set up a Web cam trained on Mei Xiang so visitors can observe the panda.
Smith said it is difficult for giant pandas to get pregnant because females only ovulate once a year for two days.
Mei Xiang and her mate Tian Tian are a little bit romantically challenged, Smith said, so Mei Xiang was inseminated with Tian Tian's sperm in January.
In 2005, Mei Xiang gave birth to her only cub, Tai Shan.
Celebrity Families of 2014 [PHOTOS]