A married couple in the US city of Denver were horrified after a doctor at an IVF clinic informed them that they were biological twins.
The couple had visited the clinic as they were having trouble conceiving, with the discovery made after a lab assistant noticed a striking similarity in their DNA samples.
Further analysis confirmed that the two were not only related, but actually brother and sister.
When a doctor broached the topic with the couple, it was clear they had no idea they were related.
"They burst out laughing," the doctor told the Denver Inquirer. "The husband said a lot of people remarked on the fact that they shared the same birthdays and looked similar, but that they were definitely not related. The poor man had no idea."
As the news slowly sunk in, the couple began piecing the mystery together.
The two had been orphaned at a young age, put into state care and adopted by different families. A bureaucratic error meant the families were never told that their adopted child had a twin.
The shocked couple are now reported to be considering the future of their relationship.