Mei Xiang is keeping people guessing. The panda was artificially inseminated with sperm from two other giant male pandas in April of this year. Her progesterone levels are sustained, and she’s been building a nest at the National Zoo in Washington, D.C., where she has previously given birth to four cubs (though only two have survived). And yet, no one is sure if she’s pregnant or not. That’s because giant pandas can also undergo pseudopregnancy, during which they show all the signs of a real pregnancy, even though they’re not with cub, so to speak. Pierre Comizzoli, a research biologist with the National Zoo who assists with Mei Xiang’s artificial inseminations and care, explains why it’s so hard to figure out whether or not giant pandas are pregnant.

Plus, take our Panda Pregnancy Test, and see how much you know about panda pregnancies.