The Toronto Zoo announced that Er Shun, a giant female panda on loan from China as part of a breeding program, is pregnant with twins. A fetal heartbeat was first detected during a Sept. 25 ultrasound. During an additional ultrasound, on Sept 29, the second heartbeat was discovered.

"Much to our surprise, we captured an image of a second panda fetus," said the Toronto Zoo in a press release, reported CBC News.

Watch a video featuring the ultrasound, released by the Toronto Zoo, below.

"While this is very exciting news, we remain cautiously hopeful as the next three weeks are critical. Zoo staff and panda experts will continue to monitor Er Shun," said the Toronto Zoo via Facebook.

As a part of a worldwide giant panda captive breeding program, 8-year-old Er Shun underwent two nonsurgical artificial inseminations in May. Sperm collected from Er Shun's male partner Da Mao, as well as frozen sperm from two pandas in China, was used for the procedures. Er Shun was earlier artificially inseminated in April 2014, but that attempt was unsuccessful.

Reproductive scientists say giant pandas display "delayed implantation," a situation during which gestation lasts from 87 to 186 days from insemination. Thus, until about 20 days before birth, scientists cannot tell whether females are expecting, explained CTV News.

Both Er Shun and Da Mao are on loan to Canada from China for 10 years. During this period, the pandas will spend five years each in Toronto and Calgary. Any cubs born to the bear couple will remain in Canada.

"We will continue to provide updates as they become available and we encourage everyone to follow us on this incredible journey," said zoo officials, according to the Toronto Sun.