Two identical boys are real-life 'mirror twins' as their hearts, liver and spleen are on opposite sides of their bodies.
Lucas and Louis Cooke, 22 months were born in a shared amniotic sac with their umbilical cords entwined.
After birth, their parents found they were 'mirror-image twins', which happens after a late split of the fertilised egg at around nine-12 days.
The twins' Richie Cooke, 27, said during the ITV programme Secret Life of Twins: "We discovered they were mirror-image twins after they were examined by a consultant.
"He explained that Lucas's heart, liver and spleen are on the opposite side of his body, which happens in extreme cases of mirror-image twins."
The twins' story was screened on an ITV in a programme called Secret Life of Twins.
The twin boys' embryo split into two after 10-days, and doctors told their parents if it had been a day or two later, they would have been conjoined.
Because they shared an amniotic sac, Lucas and Louis were also 'Monoamniotic' or 'MoMo' which means they share the placenta but have separate umbilical cords.
This, however, can cause a difficult pregnancy, and mother Danielle Cooke, 28, from Stockton had to be monitored three times a day.
After they were born, the twins stayed in hospital for six weeks and now, aged 22 months, are full of beans at home.
Mrs Cooke said they have their own twin language and love to dance together.
Mr Cooke added, when talking on the programme about the boys growing up, the closeness of the boys is "unreal to watch".
One twin, Lucas, has a heart condition called CCTGA - congenitally corrected transposition of the great arteries - which is a rare heart defect in which the heart's lower half is reversed.
However the father explained it doesn't cause him any problems and simply requires him to have annual check-ups.
Mr Cooke added: "There was a worry he had a more serious heart condition at first, but thankfully that turned out not to be the case.
"His other organs being on the opposite side have no effect on him whatsoever."