The 19-year-old giant panda Mei Xiang won't be having a cub this year, the National Zoo confirmed today. She was artificially inseminated by giant male panda Tian Tian's sperm in late May, but the fertilization effort was not successful.

Pandas only have a short window, 24 to 72 hours, in which to get pregnant. And while there’s usually an attempt at mating, this was the first time the zoo skipped straight to artificial insemination since Tian and Mei weren’t signaling their interest. Even if they do, mating can be tough for the animals. As assistant curator of the giant pandas exhibit Laurie Thompson explained, most pandas in zoos “aren’t experienced breeders so they haven’t learned. . . naturally and so you end up having to do artificial insemination.”

Over the summer, Mei had been exhibiting certain signs—higher estrogen levels, nest building, increased sleep, decreased appetite and cradling behavior—that made zookeepers cautious but hopeful. To safeguard the female panda's den, on August 18, they partially closed off the habitat area to visitors, and fully closed it on September 1. Zookeepers and trained volunteers began monitoring her behavior around the clock, starting on September 5. (The Giant Panda Cams are always available for online viewers.) The habitat will now resume normal operations on the 16th.

But they knew all along that it could be a false alarm: giant pandas experience pseudopregnancies. High levels of progesterone during the secondary hormone rise (the first is during ovulation) can make the female panda behave as if she is expecting, even when she’s not. Mei has had four pregnancies, and she’s been pseudopregnant six times.

A possible pseudopregnancy is just one part of the months-long guessing game of panda reproduction. Another factor is the length of gestation, which varies widely. “Unlike in humans,” the National Zoo explained, “the implantation of a fertilized egg for a panda can occur months after ovulation.” This process, which is called embryonic diapause or delayed implantation, means “the eggs start to divide and then fetal development stops. At this point, the embryo floats around in the uterus until, at a later stage, it attaches to the uterine wall and gestational development continues.” Because of delayed implantation, pandas can give birth after three months; other times, they might not even show signs of pregnancy until six months.

It can also be tricky to check for fetuses with an ultrasound. Because the zoologists don’t anesthetize pandas for the scans, the procedure can happen only when the female is willing. Even when the vets see something promising on the ultrasound, it’s still not time to celebrate yet. Because panda fetuses are so small—newborns come in at only 1/900 of the mother’s weight—it can be hard to spot them. Determining just what’s showing up on the ultrasound is tricky too: sometimes Mei’s diet of bamboo gets in the way of imaging. But if zookeepers do spot an outline of a fetus, that “doesn’t guarantee birth of a cub,” the National Zoo clarifies. “The female could miscarry or possibly reabsorb the fetus.” The latter is an “anomaly,” Thompson said, that they’re still not sure about. With Mei's hormones returning to normal levels, she either experienced a pseudopregnancy or reabsorbed an embryo after conception.

Giant pandas have lived at the National Zoo since 1972 but the veterinarians there only successfully began breeding pandas in 2005, when Mei bore Tai Shan, followed by Bao Bao in 2013. At two, Bei Bei still has a couple years at home with his mom but, like the others, he’ll be heading off to China around age four, as part of the agreement with the China Wildlife Conservation Association.

When they’re born, panda cubs are pretty helpless; they’re blind, deaf, and hairless. Unable to regulate their body temperature, they depend on their moms to keep them warm. For the cubs Mei has successfully born and raised, Thompson said, Mei “has proven to be a great mom—right away, her instinct kicks in.” Pandas can breed until their early 20s, so at 19, Mei might only have a couple more chances.