The Detroit Zoo said Tuesday that Kivuli, its giraffe, gave birth to stillborn twins.

"Many of you have been anxiously awaiting news of a giraffe birth. We are sad to tell you that Kivuli gave birth to stillborn twin calves early Monday morning," the Detroit Zoo wrote on its facebook page. 

The Detroit News reports handlers didn't know Kuvuli and her mate, Jabari, were expecting twins until the day of the birth. Twins are extremely rare in giraffes. 

The zoo's veterinarians will conduct post-mortem studies to determine cause of death.

"Animal care staff have been closely watching Kivuli for several months in anticipation of her first birth, and everyone is saddened by the loss," the zoo said.