A married couple who struggled to conceive only discovered they were twins when they went through IVF.
The unnamed couple underwent the treatment to help them have a baby.
But doctors at the fertility lab noticed their DNA was abnormally similar.
At first, lab technicians believed the pair may be unwitting cousins, but soon ruled that out because the DNA was too similar.
Eventually doctors confronted the pair, who burst out in laughter when they were asked if they were related.
A doctor at the clinic in Jackson, Mississippi, told the Mississippi Herald: “It’s just a routine thing and we wouldn’t normally check to see if there was a relationship between the two samples.
“But in this case the lab assistant involved was shocked by the similarity of each profile.”
Adding to the compelling evidence was the fact the pair shared the same birthday and that friends had remarked how similar they looked.
The doctor who broke the news described how the wife pleaded with him to admit he was joking, but he added: “I wish that I was, but they had to know the truth.”
It soon became clear that the pair were in fact unwitting siblings – both having been adopted as babies to separate families when their parents were killed in a car crash.
They later met at a university party and bonded over their similar backgrounds.
The unnamed doctor added: “The husband said that a lot of people remarked on the fact they shared the same birthdays and looked similar to each other, but he said it was just a funny coincidence and that the couple were definitely not related.”
The pair are now said to be re-considering their future together.