And their true identities would only be discovered with the help of genealogy website 23andMe.
Linda and Denice were born 31 minutes apart at the Bethesda Hospital in St Paul, Minnesota in the early hours of December 19, 1945.
And for seven decades they went through their lives with little doubt that they were meant to be somewhere else.
'People just automatically assume they got the right family,' Denice told KING-TV. 'It's a crazy thing.'
It was Denice who first submitted her DNA to 23andMe, hoping to simply find out more about her family's health history.
The genealogy website not only provides you with genetic information about yourself, but can also reveal who your closest relatives are if you so desire.
So Denice was shocked when, a few weeks later, the results arrived - and revealed her DNA didn't match any of the people she thought had been family all her life.
Denice, who lives in Eagan, Minnesota, decided to take the test a second time, but the results were the same
'Either 23andMe made a mistake, or I was switched at birth,' she realized. 'I was really supposed to be another person.'
The mystery began to unfurl when Denice's name popped up as a close relative on the 23andMe report submitted by Linda's niece, who was 40 miles away in Hammond, Wisconsin.
The results seemed to confirm suspicions long held by Linda's own daughter Michelle.
Michelle had always told her mother she didn't 'look like the rest' of her family and had once even checked public birth records.
So when she saw Denice's name listed as a close relative of her cousin on the 23andMe report, Michelle told Linda she believed she had been switched at birth.
'I did my DNA right away, because I've got to see this on paper,' Linda said.
Next to 'mother' was a name she had never seen before: Marianne Mayer.
How the women managed to get switched at birth is a question that will likely never get answered.
'I'm sure the nurses are dead that probably took care of us,' Linda said. 'We'll never know.'
But while they'll never discover the 'how', Linda and Denice are just glad that they found out about each other at all.
'I consider it a gift,' Denice said.
The pair have met several times since learning about each other in April and are slowly meeting the family they never knew they had.
That includes Mayer, who is 99 years old and now lives in a memory care home.
It has been especially touching for Linda who lost Rochelle Nielsen, the woman who raised her, at the age of 17 when she died from from cancer.
Linda and Denice have to tell Mayer the story about how they were switched at birth every time they visit, but neither minds. Nor does Mayer.
She happily laughs when the women tell her that now she has even more grandchildren.
Both women's families are now planning a massive reunion to meet each other and bond over their unlikely - and incredible - connection.
