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.