'I believe you're the father of my son': Shock as 'fun' DNA test reveals IVF nightmare

Donna and Vanner Johnson. Screenshot / ABC4

A US man and his family have been left in shock after discovering he is not the father of his son due to an IVF mix-up more than a decade ago.

The Johnson family, from Utah, decided to do a 23andMe DNA kit for "fun", but when they got their results a month later, they were devastated by what they found.

Speaking to US TV station ABC4, Vanner Johnson said he couldn't quite fathom what he was reading.

"When I looked on that page and saw mum for him and saw father unknown ... I thought, 'what do you mean father unknown? I am his father'," he said.

Donna and Vanner Johnson. Screenshot / ABC4

Vanner and his wife Donna began their IVF journey in 2007 after they were unable to have a second child on their own.

"You understand there's that possibility but it's so remote," Vanner said.

Donna said she knew something was wrong when the results read their oldest child was a half-sibling to his younger brother, through her.

The test revealed Donna's egg was fertilised by someone else's sperm during the IVF process.

"There were a lot of emotions we had to work through, including separating the love of our son, which has not changed … to the issue that we were dealing with," the heartbroken father told ABC4.

The couple waited a year before they broke the news to their son — and then began the mission to find the boy's biological father.

In a Facebook post, Vanner said the craziest thing he has ever had to do was tell a complete stranger, "I believe you are likely the biological father of my son …"

After tracking down who they believed was their son's biological father through the website Ancestry.com, Vanner was then forced to make an uncomfortable phone call.

The couple waited a year before they broke the news to their son. Screenshot / ABC4

"I say to him … I understand you are Devin McNeil, your wife is Kelly, both of you did in vitro a number of years ago, and he said 'yes', and I said 'well I have something I need to talk to you about'," Vanner told ABC4.

The two families began to piece together how the mix up may have unfolded and discovered there was a day when both couples were at the University of Utah Center for Reproductive Medicine.

They have both filed separate law suits against the IVF centre.

In a statement released to ABC4, the centre stated: "Although we cannot comment on patient cases without consent or ongoing litigation, the safety and care of our patients is our primary goal.

"If patients come to us with questions or concerns about their care, we evaluate our care and procedures and, if necessary, make changes to prevent harm from happening to other patients.

"Our providers and staff strive to provide excellent care and we constantly work to make improvements."