A woman bought her husband a DNA test for his birthday but the results left their marriage in shambles, The Sun reported.

The story, which was first shared anonymously on Dear Prudence, reveals that the mother-of-two bought her husband, who she grew up in the same small town, with an ancestry kit.

In the gut-wrenching letter the woman, who has a two-year-old son with her husband said the DNA test revealed they were paternal first cousins.

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“The news unsettles me and has left my husband's world in tatters,” the unnamed woman wrote.

The revelation has caused a chasm in their marriage and led the couple to suspect that her husband’s father was unfaithful to his wife.

“His parents have been married for 30 years and my husband has always held his mother close to sainthood,” she added.

“He won’t talk about this to me, we’re sleeping in separate rooms and it’s starting to affect our son.”

The couple have since decided to keep their discovery between the two of them.

“The secret is poisoning my marriage and I’m terrified it will blow up the rest of our family.”

Sources: The Sun, Dear Prudence