Giant panda Mei Xiang was sensitive to sound. She stopped showing an interest in food. During training sessions, she appeared less responsive than normal. Staff at the National Zoo knew something was happening.

Once she started pulling nesting material into her den, they got hopeful: Mei Xiang (pronounced may-SHONG) might be pregnant.

It had been two years since Mei Xiang, the only adult female panda at the National Zoo, gave birth to her last cub, Bao Bao. Pandas can become pregnant only for a few days each spring, so zoo vets tried to help fertilize Mei Xiang’s eggs in a medical procedure. Months later, they still weren’t sure it worked.

But on August 19, an ultrasound examination, which shows pictures from inside the body, revealed that Mei Xiang was carrying a fetus. Zoo officials at the time said they were “cautiously optimistic” about a birth.

Saturday turned out to be the big day. At about 5:30 p.m., Mei Xiang gave birth, and a tiny, pink cub was spotted on the Panda Cam. More than four hours later, Mei surprised everyone when she delivered another cub. Their sex will not be determined until next week at the earliest.

The births, coupled with Sunday’s birthday of big sister Bao Bao (rhymes with bow-wow), made for a weekend of panda-monium.

“The panda is so linked with who the National Zoo is,” said Steve Sarro, the curator of giant pandas at the zoo.

Amid all the excitement, the panda team remains on high alert. Although twins are a common occurrence in panda births (it happens about 50 percent of the time), both cubs don’t normally survive.

“It’s very difficult for the female to raise both,” Sarro said. At birth, panda cubs are blind, deaf and extremely fragile, making them very needy. Because both cubs are battling for the mother’s attention and milk, zoo veterinarians have a plan to increase the chances that both cubs will make it.

“Our plan right now is we’re doing a baby swap,” Sarro said. “We’re pulling one baby and letting her have one, and then we swap the kids. So she gets to bond and work with both.”

While a cub is away from Mei Xiang, the veterinarians help feed and care for it. The mother has begun to favor the larger cub and has become less accepting of the swapping process, leaving vets to take care of the smaller cub.

While around-the-clock efforts are needed by zoo vets to keep the cubs stabilized, it’s possible that both will survive.

“Zoo Atlanta had twins, and they’re doing very, very well,” Sarro said.

The National Zoo is one of only four zoos in the United States that have giant pandas. The others are in San Diego, Atlanta and Memphis, Tennessee. China owns every panda, but the United States has an arrangement with China to allow certain American zoos to borrow pandas to help them breed and increase their population, according to Sarro.

Not only are there not many giant pandas in the United States, but few remain in the world. Giant pandas are a threatened species, which makes the birth of panda cubs very important. According to the National Zoo, fewer than 1,600 giant pandas are left in the wild.

The National Zoo aims to help species whose populations have been dwindling to dangerously low levels. Although zoo vets work to breed many threatened species, such as the Cuban crocodile, pandas are their main focus.

“We save species. Pandas are very important to that mission,” Sarro said. “We want to make sure that these animals never go extinct in the wild.”