An extra infant passenger appeared on an Air Canada flight from Calgary to Tokyo on Mother's Day.

Canadian Ada Guan was two hours into the flight on May 10 when she had severe stomach cramps. A few hours later, the 23-year-old, who had no idea she was pregnant, gave birth to a baby girl.

Ms Guan and her boyfriend Wes Branch live in Victoria, Canada, and were going to Japan, Canadian media reported.

During the flight, as contractions began, she thought she had a bad stomach virus. She was moved to business class and put on a drip.

In an interview with Canada's Global News, Mr Branch said: "She told me 'Something fell out of me' and I lifted up her pants and I see a head, and then I heard 'Waah!'. And I'm like 'Oh my god, I think we have a kid'."

Mr Branch, who works for a moving company, also told CBC News he "passed out" briefly due to the shock.

Luckily, there were three doctors aboard their flight, and one volunteered after the crew appealed for medical aid.

The plane arrived at Narita Airport in Tokyo 30 minutes ahead of schedule after it was given priority to land.

The couple said that Ms Guan did a pregnancy test a few months ago, but it was negative. She thought she had just put on a bit of weight.

"Her belly wasn't even big," new father Mr Branch said. They named the baby Chloe.

Air Canada sent out a congratulatory tweet after the plane landed in Japan.

Happy #MothersDay! We wish mother and daughter well as they arrived in Tokyo safe and sound on AC009 today: https://t.co/BTZcwIYhlf — Air Canada (@AirCanada) May 10, 2015

Surprise births are rare, but doctors say they do occur.

chuimin@sph.com.sg