The zoo says Er Shun, one of two giant pandas on loan from China, is pregnant with two fetuses.
They say the panda underwent two non-surgical insemination procedures on May 14 and sperm was used from the other panda in Toronto, Da Mao, plus sperm from two other giant pandas based in China.
The zoo says a veterinarian detected a heartbeat on an ultrasound conducted last Friday and discovered the beating heart of a second fetus on Tuesday.
The zoo expects the babies will be born in the next three to four weeks.
They are "cautiously hopeful" and the coming weeks are critical.
Er Shun and Da Mao arrived from China in 2013.
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