WASHINGTON (AP) - Veterinarians at the National Zoo say an ultrasound examination on the female giant panda Mei Xiang can't yet determine whether she is pregnant.

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Zoo officials tweeted updates Tuesday as keepers encouraged Mei Xiang to cooperate.

Zoo scientists recently detected a rise in hormone levels that could indicate the panda is either pregnant or experiencing a "pseudopregnancy."

A pseudopregnancy occurs when a panda ovulates but doesn't conceive. Mei Xiang has had five consecutive pseudopregnancies since 2007. She gave birth to her only cub, Tai Shan, in 2005.

Over the weekend, Mei Xiang began building a nest and showing signs of a possible pregnancy.

The 14-year-old panda was artificially inseminated twice in late April. Zoo officials warn that panda fetuses don't start developing until the final weeks of a pregnancy.