Lady Louise's very rare eye condition and the major surgery she had aged just 18 months

underwent surgery when she was just 18 months old for esotropia - a rare eye condition that the royal was born with.

The now-20-year-old was born with the condition back in November 2003, and according to the College of Optometrists in Vision Development, it is the "most common type of Strabismus".

The college further explains: "Esotropia is one of several types of Strabismus, which is the condition of eye turns or deviating eyes.

"Esotropia is the most common type of Strabismus, occurring in approximately one to two per cent of the population."

Lady Louise was born four weeks early - with a premature birth being one of the likely reasons behind the condition, often referred to as "squints".

Previously speaking to the Sunday Express, the Duchess of Edinburgh said: "Premature babies can often have squints because the eyes are the last thing in the baby package to really be finalised."

The condition is said to affect two percent of the world's population.

Aged just 18 months, Louise underwent surgery to correct her vision - something that was unfortunately unsuccessful.

Speaking of her daughter, the Duchess - who also shares son James with Prince Edward - said: "Her squint was quite profound when she was tiny. And it takes time to correct it. You've got to make sure one eye doesn't become more dominant than the other."

Lady Louise then underwent a second surgery in 2014 - when she was 10-years-old.

This was successful, with her vision now deemed as "perfect" by her mum Sophie. The 59-year-old previously said of Louise: "She's fine now - her eyesight is perfect."

Lady Louise has recently started her third year at the University of St Andrews in Scotland. Although this is typically the final year for students, she will likely not graduate this academic year. This is due to her studies taking place in Scotland - somewhere where they study for four years - instead of three in the UK.