In a routine ultra-sound scan, Jayne Jones discovered that she was 27 weeks' pregnant and her baby was growing in her abdomen, on the omentum - the layers of fat that cover the bowel.
Almost all such foetuses die within weeks, even days, of conception and only one similar case has ever been reported in Britain.
Ten days after the scan Jones was rushed to Derriford Hospital, Plymouth by her husband Graham because she had collapsed in pain.
She had to have groundbreaking surgery that had never been done before in Britain and involved 36 NHS staff.
Billy was born weighing 2lb 2oz on April 19 and immediately put in an incubator - where he was kept warm inside a plastic bag.
"He was so tiny. He was in a little resealable sandwich bag to keep his temperature up. They told me that, for all the millions spent on the NICU, what's made the biggest difference to survival rates are Tesco resealable sandwich bags," the Telegraph quoted Jones, as saying.
"You look along the incubators and they're all in them, these bags saying Tesco. It's incredible."
"He's our little miracle baby. That's what they called him at the hospital - a miracle, Billy the Whizz," she added.
However, the birth was also dangerous for the mother.
Jones had a one in five chance of dying in childbirth and was placed in a High Dependency Unit after the birth.
She still may need another operation to remove the placenta that was left in her body but doctors hope that it will shrink inside her and be absorbed by the body.
"He's got no idea of the trouble he caused. But I'll make sure he's well aware in later years." Jones said.