According to the Daily Mail, Fergie said: She had scoliosis. In other countries in the world she would have been put in an asylum and locked up."
The 28-year-old royal was diagnosed with the spinal condition when she was just 12-years-old and underwent major surgery to correct the curve in her back.
Last month, the princess proudly showed off the scar her surgery left her with by opting for a low back wedding gown.
Reflecting on the wedding day, Fergie said: "I was never more proud to see my tall, beautiful, upstanding daughter full of courage."
Back in 2002, when Eugenie was originally diagnosed with scoliosis, Sarah told the Mail on Sunday: "We thought it was a small curvature but it was mammoth.
"Her bones had oscillated to such a degree that by the time she reached 18 she would have been a hunchback. It was horrendous."
Eugenie had 12-inch metal rods inserted into her back to fix the curve of her spine.
Earlier this year, Eugenie shared X-rays of her back for the first time to raise awareness for the condition and "honour the incredible staff at The Royal National Orthopaedic Hospital who work tirelessly to save lives and make people better".
Sharing the touching image on International Scoliosis Awareness Day, the newlywed is now a patron of a £15million project to build a new block for the North London hospital.