Is she, or isn’t she?

The National Zoo has entered the height of its annual panda pregnancy watch, in which teams of zoo pandaphiles keep a 24-hour-a-day vigil to see whether the female giant panda, Mei Xiang, might give birth to a cub.

Mei’s reproduction cycle is running a little late this year, which may or may not mean anything. Last year the zoo discovered she was not pregnant in late April. The year before, it was late May.

But in 2008 the discovery was made in August, and in 2007 it was early July.

Mei has been artificially inseminated each year since the year after her only cub, Tai Shan, was born in 2005. Tai Shan was sent to a breeding program in China last year.

This year Mei was inseminated twice in January after she went into heat. The panda gestation period usually lasts 90 to 185 days.

But giant panda reproduction is mysterious. Female pandas can exhibit many signs of pregnancy without being pregnant. And experts often must simply wait until the panda’s cycle ends — with or without a cub.

Mei went on 24-hour watch Monday.

She spends more time in her den and is eating very little, the zoo says. Both could be signs that she is pregnant, or undergoing what is called a false or pseudopregnancy.

The zoo has been attempting to perform ultrasounds several times a week since early June. But Mei has only been cooperating intermittently, “picking and choosing which times are agreeable to her,” the zoo says in a posting on its Web site. “There have been no significant findings thus far.”