A Canadian woman's surprise delivery of a baby on a Calgary-to-Tokyo flight is unusual but not unheard of, a doctor of gynecology and obstetrics said.

Ada Guan, 23, gave birth to a baby girl on an Air Canada flight on Saturday with the help of the flight crew and a doctor on board. Guan and her boyfriend, Wesley Branch, say they had "no idea" she was pregnant, up until the time she started experiencing stomach pains on the flight.

Dr. Doug Wilson of Alberta Health Services say patients of all body sizes can carry a baby to term, without recognizing the symptoms of a pregnancy or feeling the movement of the baby.

"They're just not expecting to get pregnant," Wilson told CTV's Canada AM on Wednesday. Wilson, who is head of the obstetrics and gynecology department for Alberta Health Services, said about half of all pregnancies are unexpected, and some people simply aren't paying attention to the obvious signs.

"They just don't understand or appreciate the changes that could be happening," he said. "When they start to miss their periods they don't appreciate or even think they're pregnant."

Guan told CTV Vancouver she suspected at one point that she might be pregnant, but a test came up negative so she forgot about it. "I just thought I was gaining a little bit of weight," Guan told CTV Vancouver by Skype from Tokyo.

Branch said he also noticed the weight gain but dismissed it after the false negative. "I had a feeling because her belly button was popping out and she had this weird line on her stomach," he said.

The pair had no idea what was happening when Guan started going into labour on the plane.

"She was going through the most excruciating pain that I've ever seen a woman go through," Branch said.

Flight attendants moved Guan to first class and a doctor helped her give birth.

The doctor initially told Branch it would be a miscarriage, but was surprised when the baby started breathing, Branch said.

"The doctor said there was no signs of life. And all of a sudden, there was life," he said.

The child weighed seven pounds, eight ounces at birth.

Guan and Branch are still in Japan with their newborn baby, Chloe, and are now trying to get the child a Canadian passport and pay their hospital bill. Branch says he is unemployed and on the hook for about $3,000 in expenses.

"We can't leave until that's figured out," he said.

The couple has set up a GoFundMe page to help pay their medical expenses.

With files from CTV Vancouver