A Mississippi couple who were trying to conceive discovered they were brother and sister after DNA tests at a fertility clinic.
A doctor at the IVF centre in Jackson, told the Mississippi Herald how the information came to light.
"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," he said.
At first, it was thought the man and woman, who cannot be named for patient confidentiality reasons, were first cousins. The lab assistant then realised the DNA match between them was very close.
A more detailed investigation was carried out in which it was revealed that the couple had the same birthday and were both born in 1984. The doctor came to the conclusion that they were biological twins – but the couple were completely unaware of this.
After asking them to come in for an appointment, the doctor tried to break the subject gently and asked if they were aware that they were twins. However, they "burst out laughing".
The 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."
It was revealed that their biological parents were killed in a car crash and the two children were separated and adopted by different families.
The pair had later met at college and became close.
"They were attracted to each other due to their similarities," the doctor said. "The fact that they'd both been adopted after their parents had died, meant they'd both experienced a similar childhood, and they felt they could really connect with each other. If they had only known the truth, it could have saved them so much pain later on."
The couple are now considering their future relationship. In Mississippi, marriage between siblings is illegal and can be penalised with a 10-year-prison sentence. However, the maximum fine is only $500.
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