Courtesy Annie Johnston

Annie and Joby Johnston of Lewis Center, Ohio, decided last February to take a break from the surgeries, in-vitro fertilization and artificial insemination they'd gone through for five years to try to have a baby.

"We were tired of doing the same thing and expecting a different result," Annie, 34, told GoodMorningAmerica.com. "Chrissy was our different thing this time around."

Chrissy is Chrissy Knott, Annie's only sibling and older sister, a 37-year-old married mother of two boys.

Knott told her sister and brother-in-law in 2011, after their first few rounds of in-vitro failed, that she would carry their baby for them. Two years later, on Feb. 19, the Johnstons took her up on her offer and a doctor implanted two embryos in each sister.

"The doctor made sure that everyone was OK with the possibility of four babies because it was possible," Johnston said. "But, for us, after trying for so long, we were just trying to increase our chances of getting at least one."

Nine days later, the two sisters found out that they were both expecting a baby. One month after that, at the seven-week ultrasound, the two sisters learned they were each carrying twins, classified technically as quadruplets.

"I was relieved, more than anything, that the babies were still developing," Johnston said. "My husband, I think, was thinking more down the road of what we were going to do with four babies."

Joby Johnston didn't have too much to worry about, fortunately, in terms of helping hands considering the two sisters live only seven houses apart. The sisters' parents, upon finding out their family was growing by four, put their house on the market and moved from nearly two hours away to live with the Johnstons and prepare for the babies.

On Oct. 24, Johnston and Knott gave birth to four healthy babies in scheduled C-sections so that the quadruplets would share the same birthday.

"My sister was carrying the two boys, and she delivered first, so I could be there in the room for her when she had them," Johnston said. "Two hours later, at 9:30 a.m., I delivered the girls."

The babies - Charles (6 pounds), Thomas (6 pounds, 2 ounces), Grace (5 pounds, 7 ounces) , Hadley (5 pounds, 4 ounes) - were all born healthy.

"We still can't believe it sometimes," said Johnston.

Johnston said the family had transformed the great room of their four-bedroom house into a makeshift nursery until the babies were big enough to sleep in their cribs. She and her sister are both pumping breast milk and feeding the babies with bottles, which allows "more hands to help," said Johnston.

Once the babies are big enough, they'll move upstairs into their nursery, and eventually be told by their mom and aunt the happy tale of how they came into the world.

"We're very thankful for what she did for us," Johnston said of her sister. "We're very grateful and thankful that they're all here."