Kristine Casey is not just any grandmother to her grandson, Finnean. She is also his surrogate mother.

Casey, then 61, gave birth to Finnean in February 2011 after her daughter, Sara Connell, struggled with infertility. Connell's egg and husband Bill's sperm were used in the in vitro fertilization procedure, making the couple Finnean's biological parents, and Casey the gestational carrier of their child.

"The idea, we never could have fathomed," Connell, 36, said today on " Good Morning America." "I felt so connected with Finn and with my mom and yet it was a completely surreal, really fantastic situation."

Connell tells the story of their family's unconventional journey to motherhood in a new book, " Bringing in Finn: An Extraordinary Surrogacy Story."

Connell struggled for years with infertility before her mother stepped in as a surrogate. "My husband and I had always wanted children," Connell said on "GMA."

"We were ready to start a family. I came off the birth control pill and I wasn't having a cycle, so we tried holistic treatments, acupuncture, yoga and then went to a fertility specialist who said, 'You're not ovulating, you're likely going to need help having a child.'

"We moved onto fertility treatments, IVF, I lost twins at almost the third trimester, late into the pregnancy, which was really hard. Then we got pregnant one more time and had a miscarriage," she recalled.

Casey, who had given birth to Connell and her two sisters 30 years earlier and gone through menopause 10 years ago, offered to act as the surrogate.

After months of tests and soul searching, they decided to go ahead with in vitro and after the second IVF cycle, Casey became pregnant.

When it comes to surrogate parenting, the Connell's arrangement is not as uncommon as one might think. In August, 49-year-old Linda Sirois of Maine gave birth to her grandson Madden when her daughter Angel Herbert, 25, son-in-law Brian Herbet where unable to conceive.

While age is a limiting factor for the safety of such late-in-life surrogacy, hormonal supplementation and the use of donor eggs make pregnancy possible even in women who have gone through menopause. ( Click here to read more on the medical aspects of late-in-life pregnancies).

"The doctors were very clear that the percentages [of complications] did increase with my age being a factor, but still the odds were pretty overwhelming that we would be successful," Casey said today on "GMA." "I just felt like it was a journey we needed to take."

Casey, who likened carrying Finnean to babysitting for nine months, said the late-in-life surrogacy was "amazing."

"It was so amazing to feel that little heart beat and the little movements inside of me," she said. "To feel, the confidence, for some reason, I felt confident I could do this and we could have this wonderful grandson."

Click here to see photos of Casey and Sara through the pregnancy.

Nine months later, Casey delivered the greatest gift a mother could give - a healthy, 7-pound boy who has grown from baby to toddler - and created an infinite bond between a mother and daughter.

"My gratitude really can't even be described in words," Connell said.

Connell, a writer and life coach, lives with husband Bill in Chicago, Ill., where they are raising Finn. Casey lives in Alexandria, Va., but the families visit each other often. Although they have not decided when or how they will tell Finn about how he came into the world, Connell says it's something they celebrate.

"This is really something we want to celebrate in our family," she said. "It feels like a miracle that we got to really witness. So when he's old enough to understand those things, we'd love to find a way to share what the experience was like, but it's also just how he got here and his life is his own now."

CLICK HERE to read an excerpt of "Bringing in Finn: An Extraordinary Surrogacy Story."

ABC News' Courtney Hutchison contributed to this report.