One of the Toronto Zoo’s giant pandas is pregnant — with twins.

Zoo staff and breeding experts observed two separate, fetal heartbeats in ultrasounds taken in the last week of Er Shun, a female giant panda on loan from China.

The next three to four weeks are critical, the zoo said in a statement, and “staff are cautiously hopeful for healthy births within this time frame.

Toronto spokesperson Jennifer Tracey said the support from people in Toronto and colleagues from other zoos around the world has been overwhelming.

“It’s an incredible accomplishment,” she said. “It’s great news for our reproductive program and for the endangered panda species.”

Er Shun was artificially inseminated last May. Giant pandas have a gestation period that can last anywhere from 87 to 186 days from the date of insemination, the zoo explained.

The first heartbeat was observed in an ultrasound on Sept. 25 and a second panda fetus was seen on Sept. 29.

Tracey said people don’t comprehend the complexities involved in this type of pregnancy.

“There are so few facilities that house pandas in the first place, and even fewer with reproductive programs,” she said.

Veterinarian Dr. Cathy Gartley from the Ontario Veterinary College and the University of Guelph has been driving in from Guelph to perform regular ultrasounds on Er Shun.

“This is the earliest pregnancy evidence recorded,” she said. “These ultrasound pictures are pretty precious.”

Gartley explained that pandas have delayed implantation, where the embryo “just sort of hangs out” for one to 10 months. This makes detecting pregnancy difficult.

She said the hardest part is finding a willing patient, and that Er Shun’s co-operation and good behaviour helped in capturing the images.

She said she bribes Er Shun with apples and does her work while she’s munching away.

“You go through a lot of apples and a lot of time,” she laughed. “Last Friday, we ran out of apples and I didn’t see that she was having twins.”

Gartley said she guesses the cubs will be born mid-October, but that hormone levels and behavioural changes are still being tracked to determine an exact window.

With three possible fathers, the paternity will not be known until the twins are born. The other giant panda at the Toronto Zoo, Da Mao, is one possible dad-the others are pandas from China.

“My money’s on Da Mao,” Gartley said. “He had just hit puberty when we took his sample. He’s got the goods.”

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If the twins survive they will be nursed by their mother for 18 to 24 months before they are weaned. Tracey said the cubs will remain in Canada.

Er Shun and Da Mao arrived from China in 2013 and are slated to move to the Calgary Zoo in 2018.

Gartley said everyone at the zoo has their fingers crossed for two healthy births.

“I’m looking forward to pictures of babies-not just fetuses.”