Congratulations are in order for the National Zoo's female giant panda—or so veterinarians hope based on an ultrasound yesterday: Mei Xiang is presumably pregnant. Experts are cautious as the 1.5-inch-long fetus could still be reabsorbed or the panda could miscarry. Research vet Pierre Comizzoli says the ultrasound may even have detected bamboo fiber. But a baby for Mei Xiang, who's given birth to just two healthy cubs in the last decade, would be big news, reports Smithsonian. As researchers have been detailing over months using #PandaStory, the artificial insemination of a giant panda is incredibly difficult and frustrating. Female giant pandas have just 36 hours per year to be impregnated. Should a pregnancy occur, a panda embryo stalls its development for three to five months so it can be delivered at the ideal time, a process known as embryonic dispausal or delayed implantation.
Afterward, it spends 45 to 50 days in the womb. During that time, experts usually identify a pregnancy by a spike in the mother's progesterone levels. Vets confirmed earlier this month that Mei Xiang—artificially inseminated in April with fresh and frozen semen, reports Phys.org—has high levels. "So she's going to start to build a nest and she's going to start to stay in her den for longer periods of time," says Comizzoli. But pandas also have phantom pregnancies—Mei Xiang has had six—meaning when progesterone levels decrease, indicating a baby should be born within days, no cub appears. Ultrasounds, if pandas allow them, are also unreliable because cubs are so small—less than a pound—at birth. The ultrasound yesterday has experts crossing their fingers. The zoo says Mei Xiang "could give birth early next week, or possibly in early September," per NBC News. (One crafty panda faked a pregnancy for extra food.)