Staff at the Calgary Zoo say they are saddened after Sparky the hippopotamus delivered stillborn twins Friday afternoon.

The 26-year-old river hippo had been on birth control for several years, so staff didn't think it was possible for her to become pregnant. However, Sparky defied the odds – but ultimately staff were not able to resuscitate the stillborn calves.

"Twin births are extremely rare in hippos and the likelihood of both surviving would have been small, but we would have liked the chance to help Sparky try to raise them," said Colleen Baird, curator of the Savannah exhibit at the Calgary Zoo. "This is very, very sad for our team."

Zoo staff say a necropsy revealed the nearly full term fetuses had died just prior to delivery, likely due to twisted or entangled umbilical cords.

It's not clear whether the zoo will put Sparky back on birth control but Baird says they are focused on keeping Sparky healthy.

"Our priority now is making sure that Sparky is alright," she said.

This was Sparky's fourth pregnancy.

Her first calf in 1991 was also stillborn.