Updated August 23: Mei Xiang gave birth to a second, apparently healthy cub at 10:07 p.m. on Saturday evening. It was not totally unexpected; pandas give birth to twins about half the time. The twins are alternating between mother and incubator, as the mother can only care for one at a time.

The Smithsonian National Zoo has detected a possible fetus in an ultrasound of its giant panda , Mei Xiang . This is a first for the zoo’s panda breeding efforts.

The discovery of what veterinarians think is a 1.5 inch (4-centimeter) long fetus means Mei could give birth as early as next week, according to a statement released after the August 19 ultrasound.

"Our success at identifying a fetus during this week’s ultrasound procedure was really the result of preparation and hard work colliding with opportunity," Don Neiffer, the zoo's chief veterinarian, says in an email.

Mei has rarely cooperated with past ultrasounds, and when the zoo staff called Mei to participate in the August 19 examination, "we were not expecting her to hold for us," Neiffer says. "When the keeper staff performed their magic, it made my job much easier."

An ultrasound only shows a panda fetus within the last 20 days of a pregnancy, and even then, "it’s just not easy to hit a moving target with the ultrasound probe," Neiffer says. "Thank you, Mei Xiang!"

The zoo's reproductive team artificially inseminated Mei Xiang in late April with semen from two males: The other National Zoo panda, Tian Tian, and a Chinese panda named Hui Hui, which was determined to be a good genetic match for Mei. If there is a cub, DNA analysis will determine the father.

Breeding the endangered animals in captivity is fraught with challenges. We talked to three experts to find out exactly what's so tricky about pandas' reproductive biology.

Female pandas ovulate just once a year, in the spring.

What's more, the female can only conceive for about two or three days around ovulation, which means she has to mate with a male during that period. Females can be fertile between the ages of about 4 to 20. (See “Is Breeding Pandas in Captivity Worth It?”)

Generally solitary creatures, panda partners find each other in the wild via calls and scents, according to the National Zoo. Luckily for captive panda pairs, the male's nearby, but that doesn't mean mating is easy. Which is why ...

The pair has to be compatible.

Not only do they need to get along, the couple has to be "behaviorally competent"—meaning males need to know how to mount a female, San Diego Zoo panda veterinarian Meg Sutherland-Smith said in a previous email interview. But sometimes pandas just aren't sure what to do. ( Read about a Thai zoo that turned to "panda porn" to get its bears in the mood .)

The San Diego Zoo's panda pair, Bai Yun and Gao Gao, have mated naturally and are parents to several cubs.

At the National Zoo, Tian Tian and Mei have had trouble mating naturally, but Mei has conceived before via artificial insemination. Tai Shan, born in 2005, was affectionately nicknamed Butterstick among D.C. fans due to a newborn panda's tiny size, and now lives in China. ( Watch a video of Mei's first baby, Tai Shan .)

Zoo Atlanta's pandas Lun Lun and Yang Yang have also failed to mate naturally, "probably because both lack an experienced partner," Rebecca Snyder, formerly Zoo Atlanta's curator of mammals, said previously by email.

Artificial insemination at Zoo Atlanta has produced several cubs.

Chinese breeding centers have also been successful at using artificial insemination, Snyder noted.

In China, panda bloodlines are also filed so that experts can pair pandas that are unrelated, which increases their chances of breeding and producing healthy offspring.

Pandas have pseudopregnancies.

View Images Veterinarians detected what they believe is a 1.5-inch (4-centimeter) long fetus in an ultrasound of Mei Xiang's uterus on August 19. Photograph by Smithsonain National Zoo

A hormone spike in a female panda tells scientists that one of two things will happen in 40 to 55 days: She'll either give birth, or experience the end of a pseudopregnancy. ( Read about the costs of breeding pandas in National Geographic magazine .)

According to the San Diego Zoo blog , "a pseudopregnancy occurs when a female exhibits the signs and symptoms of pregnancy when in fact she is not experiencing one."

The phenomenon—still not completely explained—has been observed in mice, dogs, and even people.

A panda fetus is extremely slow-growing, making it hard to detect pregnancy.

Giant pandas undergo what's called "delayed implantation," which means that once the egg is fertilized, everything gets puts on hold. The future fetus "virtually stops growing in the uterus and instead free-floats without growth until the time is right for it to implant in the uterine wall," the San Diego Zoo’s blog explains.

That's usually within the last weeks of gestation, so zoo veterinarians often don't know if a panda is pregnant until the little girl or guy pops out.

Not to mention giant pandas sometimes resorb, or absorb, a fetus—a biological process that's still a mystery to scientists.

However, ultrasound technology has improved in recent years, Sutherland-Smith noted, and her team has been able to both detect pregnancies and predict roughly when Bai Yun will give birth.

Cubs are very needy and vulnerable at birth.

At three to five ounces, the hairless, blind newborn panda is 1/900th the size of its mother. According to the National Zoo, that makes it one of the smallest mammal newborns relative to its mother's size, trumped only by marsupials. Unable to move, the infant is reliant on the mother's warmth, milk, and protection to survive.

Mei's second cub, a female, was born in September 2012, but died a week later due to lung and liver damage, according to the zoo. Poorly developed lungs likely prevented the cub from getting enough oxygen.

Saving Wild Pandas

Ultimately, "captive breeding can only take us so far, and its greatest contribution isn't the birth of a single panda but the arousal of interest and support for panda conservation at large," Marc Brody, senior adviser for conservation and sustainable development at China’s Wolong Nature Reserve, said in a previous interview. (Also see " Wild Panda Population Up Dramatically in China, Government Says .")

Possibly as few as 1,600 giant pandas still roam the mountainous forests of central China, according to the National Zoo, and more than 300 live in breeding centers around the world. (See more panda pictures.)

Some of the captive pandas in China have been reintroduced into their native habitat, "but the bigger question is not can we breed an animal in captivity, but can we give him a home in the wild—and that means restoring degraded and fragmented habitat," said Brody, who is also a National Geographic grantee.

Zoo Atlanta's Snyder agreed.

"Captive breeding is just one tool to help conserve giant pandas," Snyder emphasized. "The wild population and its habitat must also be protected. I don't want pandas to only be found in zoos in the future."