Call her the ultimate stowaway.

A baby girl born on a Calgary-to-Tokyo flight came as a complete surprise to everyone onboard, including the Canadian mother who had no idea she was pregnant.

Ada Guan, 23, started experiencing stomach pains during an Air Canada flight from Calgary to Tokyo on Saturday. Flight crews moved her up to first class and a doctor on board stepped in to help. Much to everyone's surprise, the doctor discovered Guan was pregnant.

With help from the doctor and flight attendants, Guan gave birth to a baby girl in the first class section of the airplane some 35,000 feet over Russia, with her boyfriend, Wesley Branch, at her side.

"This happened completely unexpectedly," the new father told Japan's Fuji News Network after the flight.

No one knew Guan was pregnant, Branch's mother, Sandra, told CTV Vancouver. She said her son called her to deliver the shocking news on Mother's Day. "You're a grandma," he told her.

"I said, 'What, how could this happen? She wasn't pregnant!'" Sandra Branch recalled. "Well, it turns out she was."

Guan and her new baby, Chloe, were taken to hospital from the airport and are doing well.

Doctors estimate the baby was born about 37 weeks into the pregnancy.

Most airlines advise against flying after the 36th week of pregnancy, but in this case, no one knew Guan was carrying a tiny stowaway in her womb.

Despite arriving into the world in Russian airspace, the baby will be considered a Canadian citizen. Immigration lawyer Raj Sharma says baby Chloe will be a Canadian citizen because she was born to Canadian parents. "That child is not going to be a Japanese citizen or a dual citizen," Sharma added in an interview with CTV Calgary.

Guan and Branch are from Victoria, B.C. and have been dating for about a year. They were headed to Japan for a vacation.

With files from CTV Calgary and CTV Vancouver