Teams of fans are keeping a 24-hour-a-day watch on the National Zoo's giant panda, Mei Xiang to see if she might give birth.
Scientists at the zoo detected a rise in hormone levels in May that could indicate that by early July she may give birth or end a false pregnancy.
Mei Xiang was artificially inseminated twice in January. But it's difficult to know whether a panda is pregnant or exhibiting signs of a false pregnancy.
Since early June, zoo veterinarians have been trying to perform ultrasounds several times a week. The zoo says Mei Xiang has not always been cooperative. She is also exhibiting signs of a pregnancy, such as eating less, sleeping more and cradling objects.