Mark MacDonald grew up knowing he was adopted after his biological mum had him at just 16, but he says he "never really gave it much thought".
In fact, Mark never bothered to look into his lineage until he and his wife Tina discovered they weren't able to safely conceive a child.
After receiving the devastating news, the 49-year-old professor from Portland, Oregon, began searching for his birth parents, PEOPLE reports.
Mark told the publication he wanted to find the connection he'd hoped to get through having children.
At the same time as he searched for his birth mother, Mark and Tina also looked into surrogacy options.
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Originally from Canada, Mark applied to find his birth mother through the Canadian government, a process he described as "slow."
But one day, he got the call he'd been waiting for - his mum had been tracked down and she wanted to meet him. He also found out she was now married to his birth father, and the couple had had three more children.
Meanwhile Rachel Elliott, Mark's biological sister, had been thinking about tracking down her long-lost brother - and had known he'd existed for several years after her mum told her the story.
When she learned Mark had applied to meet her and the family, she couldn't wait for him to make contact and sent him an email.
According to Mark, the pair immediately had an "extraordinarily intense" connection and began emailing regularly.
"I became this obsessive teenager-type doing nothing but checking my email and sending responses all the time," Mark said.
After seven weeks of chatting with his relatives online, Mark took Tina to Kentucky to meet them in the flesh for the first time.
"They were still largely strangers to me, but I'd never felt so comfortable," he said, adding they "fit me like a glove."
Just over two months later, Rachel flew over to Oregon to see the couple again - and she made them an incredible offer that she'd been thinking about for a long time.
On the last night of her visit while they sat down to dinner, Rachel offered to be a surrogate for Tina and Mark - and the pair accepted the offer without hesitation.
And in 2008, Mark's twin daughters Alaska Rachel and Zoe Rachel were born - named after the aunt who carried them.
Now the girls are in their teenage years, Rachel and Mark have decided to write a book about their amazing reunion and the selfless act Rachel did for her brother.
The book, Love and Genetics, was released on 22 March and took a decade for the pair to write.
Mark says the main reason he decided to write the book was so his daughters could learn more about their family - and while one of the girls has given her dad editing notes on the copy, the other is taking her time until she's ready to read it.
While Rachel is used to people telling her she gave her brother a "beautiful gift" she argues she's the one who got "the most amazing gift of all."
She said: "I got a brother."
Did you meet your long lost family? Email jessica.taylor@reachplc.com
