How Panda Pregnancies Can Be a Months-Long Waiting Game

Beginning Aug. 18, the David M. Rubenstein Family Giant Panda Habitat will be partially closed to keep a quiet area around panda Mei Xiang’s den. Although we are not able to confirm if she is pregnant, Mei Xiang is exhibiting expected and normal behaviors after the secondary hormone rise that are in line with both a pregnancy and pseudo or false pregnancy. She is building a nest in her den, has a decreased appetite, is sleeping more and reacting to loud noises. Paws-crossed!

Pandas, like several other species, can undergo pseudopregnancies, where they do everything they would if they were pregnant. At the end of a pseudopregnancy, however, hormone levels return to baseline and females’ energy levels and behavior return to normal.

The closure will not affect the outdoor habitats and viewing areas. Please note, that although Bei Bei will have access to be outside until 2 p.m., due to the weather, the best time to see him will be outside in his yard from 8 a.m. to 10 a.m. Indoor viewing for Bei Bei will be closed.

On May 25, Smithsonian Conservation Biology Institute scientists, the Zoo’s veterinarians and colleagues from the China Conservation and Research Center for the Giant Panda artificially inseminated an ovulating Mei Xiang. Unlike in humans, the implantation of a fertilized egg for a panda can occur months after ovulation. It’s a phenomenon referred to as embryonic diapause or delayed implantation. Here, 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.

While it’s unclear why delayed implantation occurs, the phenomenon isn’t unique to giant pandas. In fact, hundreds of mammals including kangaroos, skunks, and otters experience it. According to Pierre Comizzoli, a research biologist at the Smithsonian Conservation Biology Institute, it’s likely a way for females to adapt to the environment and make sure they’re going to give birth at a time when there will be enough resources for the female to eat and subsequently produce milk for her offspring.

Still, it’s this delayed implantation that’s responsible for the variation in panda pregnancies—some embryos implant shortly after fertilization and pandas give birth in as little as three months; other times it can even take six months to see signs of a viable pregnancy. With the implantation comes a surge in progesterone—which scientists refer to as a secondary hormone rise. “That phase is actually much more predictable and lasts between 40 and 50 days,” says Janine Brown, reproductive physiologist at the Smithsonian Conservation Biology Institute’s Center for Species Survival. During this time, the female also starts nesting in preparation for a cub. In addition, she becomes less active, spending most of her time inside her enclosure.

At that point, scientists will step up their daily monitoring of progesterone to track when the hormone levels return to baseline—a change that signifies the gestation period is over. “If it comes down to baseline and you don’t see a cub, it means she wasn’t pregnant or she lost the pregnancy,” says Brown.

To complicate matters further, pandas—like dogs and mice—also experience what’s referred to as pseudopregnancies or false pregnancies. “It’s a specific term that means that an animal, for all intents and purposes, looks like she’s pregnant,” says Brown. A panda experiencing pseudopregnancy displays the same behavioral and physiological changes—such as decrease in appetite, nesting, and swelling—that she would if she were truly pregnant.

It’s still too early to tell if Mei Xiang is pregnant or experiencing a pseudopregnancy, but the panda team will continue to monitor her behavior, hormones and changes in her uterus.