A married couple who were struggling to conceive were revealed to be biological twins after a routine DNA test at a fertility clinic raised suspicion.
The man and woman, who cannot be named for confidentiality reasons, were hoping to try IVF but the lab assistant noticed an abnormal similarity between their DNA profiles.
A doctor at the clinic in Jackson, Mississippi - who also cannot be named - revealed the astonishing events to the local newspaper and explained how the situation had come about.
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"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 told the Mississippi Herald.
His first reaction was that they might be first cousins - until he noted there were far too many similarities in their DNA results.
After realising the man and woman both had the same birthday in 1984, the doctor decided they must be biological twins but didn't know if they were already aware of this or totally oblivious.
When the couple next came for an appointment at the clinic, the doctor says he tried to approach the subject gently. He asked them if they knew they were twins and says they "burst out laughing".
"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," he said.