New York (CNN) Anias and Jadon McDonald are twins conjoined at the head. Their birth was rare; science says the boys are one in millions. Their parents couldn't agree more.

"They're so perfect," says their mother, Nicole. "They're beautiful, and they are so funny and so happy."

At 13 months, Jadon is the rambunctious one. He runs in place and tries to roll off the bed to get away from his brother. When his brother plays with a toy, Jadon rocks back and forth until he can snatch it away.

Anias is the contemplative one who loves listening to his mom read. He's struggled more in infancy than his brother. He is the silent warrior, his mother says, with a gaze so captivating, it's as if "he looks at your soul."

"I could almost keep them like this," Nicole says.

Their father, Christian, agrees. He says they've grown to know both boys' personalities, and "we like them the way they are."

And yet Nicole and Christian have made the decision no parent should ever have to make: do what they believe is in the best interest of their twin boys, even if it means risking their lives.

"This is so hard," Nicole says, "I'm not going to sugarcoat it."

She says she's even started "going through a grieving process, because I've only known them" joined together. She runs her fingers through a swirl of hair the boys share at their foreheads and says how much she'll miss doing that.

The couple is sustained by faith. They pray often and seek counsel from their minister. They say they've put their trust in God, no matter the outcome. "I don't know what the answer is to my prayers yet," Nicole says, breaking down crying.

Christian says he recently began to wonder whether they're doing the right thing. Family and friends reassured him that they're making the proper decision.

"I can't wait to see them as two separate little boys," he says. "That's what excites me the most. I really just want to know my boys."

Photos: Conjoined twins at the head prepare to be separated Anias and Jadon McDonald were born conjoined at the head on September 9, 2015, via an unscheduled C-section. "They were normal little boys, like any other two little babies you would see," said their father Christian McDonald, "except for being conjoined." Hide Caption 1 of 13 Photos: Conjoined twins at the head prepare to be separated "Don't worry," Nicole McDonald said to her husband when she told him about being pregnant with conjoined twins. "We'll figure it out." Hide Caption 2 of 13 Photos: Conjoined twins at the head prepare to be separated "I can't wait to see them as two separate little boys," said Christian. "That's what excites me the most. I really just want to know my boys." Hide Caption 3 of 13 Photos: Conjoined twins at the head prepare to be separated The parents describe Jadon, left, as the rambunctious one, and Anias as the contemplative one who loves listening to his mom read. Hide Caption 4 of 13 Photos: Conjoined twins at the head prepare to be separated When the boys were younger they could be transported in a kid's wagon, seen here when they were three months old at Rush University Medical Center in Chicago. Hide Caption 5 of 13 Photos: Conjoined twins at the head prepare to be separated Anias stares into the camera. His gaze is so captivating, his mother says, "it's like he looks at your soul ... He's just a silent warrior. He's sweet and strong." Hide Caption 6 of 13 Photos: Conjoined twins at the head prepare to be separated Nicole and Christian McDonald at their home in the Bronx. Hide Caption 7 of 13 Photos: Conjoined twins at the head prepare to be separated The boys also have a 3-year-old brother, Aza. He's grown more quiet in recent weeks ahead of the twins' surgery. Hide Caption 8 of 13 Photos: Conjoined twins at the head prepare to be separated The boys share 1.5 to 2 inches in diameter of brain tissue. The surgery to separate them could take as long as 20 hours. Hide Caption 9 of 13 Photos: Conjoined twins at the head prepare to be separated Only one out of every 2.5 million live births results in twins conjoined at the head. But 80% of them die of medical complications by the age of 2 if not separated. Hide Caption 10 of 13 Photos: Conjoined twins at the head prepare to be separated Dr. James Goodrich, here with Nicole and the boys, is the most experienced craniopagus neurosurgeon in the world. Separating Anias and Jadon will mark his seventh separation surgery. He's consulted on 13 other conjoined twins. Hide Caption 11 of 13 Photos: Conjoined twins at the head prepare to be separated New 3D imaging and mapping has made this surgery unique and allows the surgical team to see exactly what to expect when they make the first incision. "There's no more guessing," said Goodrich. Hide Caption 12 of 13 Photos: Conjoined twins at the head prepare to be separated Christian and Nicole pose on the bed with their twins. Anias is on the left, while Jadon is facing his father. Hide Caption 13 of 13

Early Thursday, Jadon and Anias were wheeled down the pediatric corridor at Children's Hospital at Montefiore Medical Center in the Bronx. They were moved from their hospital room on the 10th floor to the operating room about 7:15 a.m. Their 3-year-old brother, Aza, rode on the gurney with them, leading up to the OR. Mom and Dad kissed them goodbye before steeling themselves for up to 20 hours of waiting, all with the hope two healthy boys will return -- together, but forever separated.

At the invitation of the McDonald family and Montefiore hospital, CNN has been given exclusive access to the remarkable and rare journey of Jadon and Anias. From the heart-wrenching decisions to move forward with surgery to the neurosurgical marvel of the operation itself to the months of recovery and rehabilitation, CNN will offer a glimpse into a world few have ever seen.

The surgeon leading the operation is Dr. James Goodrich. Considered by many to be the world's leading neurosurgeon for twins conjoined at the head, he now hopes to become the man who comes between Jadon and Anias and successfully separates them.

"This is about as complicated as it gets," Goodrich says while holding 3-D printed models of the boys' conjoined brains.

Twins joined at the head, called craniopagus twins, are exceedingly rare, occurring in one out of every 2.5 million births. About 40% of the twins are stillborn, and another third die within 24 hours of their birth. Studies have showed that 80% of twins joined at the head die of medical complications by the age of 2 if not separated.

Simply making it to this point has been quite a feat for the McDonald boys, who share 1.5 to 2 inches in diameter of brain tissue. Although meaningful statistics are hard to gather with something so rare, it is safe to say the separation procedure carries major risks, including the possibility of death or long-term brain damage for one or both boys.

Goodrich prefers not to dwell on those risks and instead describes a sort of rebirth he hopes to achieve for Jadon and Anias. "The boys will forever have a second birthday," Goodrich says, to mark the day they were separated.

"They go back to a 1-month-old," Goodrich says. "They have to learn to sit. They have to learn to roll. They have to learn to walk. They basically go through a yearlong period of a second infancy."

Nicole and Christian know well the milestones the twins have missed in their first year, like crawling and walking. They marked those with their oldest son, Aza.

Sitting on a park bench outside the hospital, Christian lays out his hopes and dreams. He longs to roughhouse with the boys and chase them. Nicole simply wants to pick them up when they cry and rock them to sleep.

The boys don't have cribs. They share a queen-size mattress with a pillow between them. There's no stroller big enough to allow for walks around the block. The same goes for car seats: When the twins are transported, Mom and Dad must place them on the floor of their minivan and drive very carefully.

It's those simple everyday moments of parenting they miss. Even the thought of pulling a T-shirt over the boys' heads can send Nicole into a fit of tears. That's something she's never been able to do. The boys wear only onesies.

Nicole McDonald says she and her conjoined twins, Anias and Jadon, are "in the final lap of this part of the journey."

The parents say they are telling their story to honor the medical staff for taking care of them over the last several months -- and they hope their message can help other parents struggling with a sick child. "To let them know these things that are difficult," Christian says, "can be a blessing in disguise."

Nicole, 31, worked as a pediatric physical therapist and has been helping the boys with their motor skills, preparing them for being separated. Her favorite thing now is sitting quietly in the living room, listening to them babble back and forth in their bedroom, as if they're talking to each other.

For Nicole and Christian, life nowadays is a collision of fear and joy, of excitement and anxiety. There are so many unknowns about prognosis and survival the McDonalds have had to find comfort in uncertainty. Christian says they understand that "one or both twins can be neurologically challenged, or they can have a mental or physical handicap" as a result of the operation.

"We know that is definitely a real possibility, but we're still going to love our boys," the 37-year-old father says.

They worry most about Anias. He has suffered with breathing and feeding difficulties and has had vision and heart issues. He is fed by a machine hooked up to his stomach. He had seizures at one point, but they've since been kept in check.

Standing in the boys' bedroom, the parents agree that despite their worries, surgery is in the boys' best interests. There have been other craniopagus twins, similar to the McDonalds, who have grown old joined together, learned to walk and even get married. It is by all accounts a challenging life. Not the life Nicole and Christian want for their boys.

"We're so excited," says Nicole.

"These boys, they're raring to go," Christian says. "They can't stay like this any longer."

Glancing over at the twins, the mother tells them, "In just a couple weeks, when you cry, I'll get to hold you."

The boys squeal.

The most experienced neurosurgeon of his kind in the world

Dr. James Goodrich enters the conference room at Montefiore Medical Center carrying an assortment of those 3-D replicas of the twins' brains.

"I've got more brains than I know what to to do with," he quips and then takes his seat.

Most recently, he separated a set of Syrian twins in Saudi Arabia this year.

Dr. James T. Goodrich in his office at The Children's Hospital at Montefiore.

Goodrich is the most experienced craniopagus neurosurgeon ever. This will mark his seventh separation surgery. He's consulted on 13 other conjoined twins -- all joined at the head -- around the world.

Pushing 70 years old now, Goodrich still has the boundless energy required of a man who will operate nonstop through the day -- and the night. His bright gray beard makes him look more like a globetrotting archeologist, which is half right, than a pediatric neurosurgeon. The lifelong surfer returned from Chile last week, chasing the big waves.

The McDonald boys have undergone three operations over the last year to begin the separation process. They've had four tissue expanders placed in their heads to expand their skin so it can be stretched and sewn back together.

"So far, everything's gone beautifully," Goodrich says.

He will be joined in the operating room by Dr. Oren Tepper, a 39-year-old plastic surgeon who will reconstruct the boys' skulls and stitch their heads closed.

He's never worked on conjoined twins before but has participated in hundreds of cranial-facial surgeries during his career. "This is a once-in-a-career case," Tepper says. "I have all the skills, all the training to be able to do this."

Dr. James T. Goodrich holds a 3D model of the McDonald's twins in his office at The Children's Hospital at Montefiore.

About a week beforehand, Goodrich and Tepper sit in the conference room for a virtual walkthrough of the surgery. The 3-D models on the table are just a small part of the technological advancements made over the past decade.

Three-dimensional computer images appear on a large screen. The visuals are piped in by a state-of-the-art company called 3-D Systems. Goodrich talks back and forth with biomedical engineers from Colorado on speakerphone, instructing them on what he would like to see onscreen.

An image of the boys' conjoined skulls appears. Then the skulls disappear to reveal their brain images in three dimensions.

"Let's put the top brain back and take the bottom brain out," Goodrich says.

Within moments, the venous structure of one child's brain flashes onto the screen. Goodrich points to a cluster of intricately tangled veins the boys share; these are his biggest concern for the operation.

"I know the vascular system we have to go through is complex," Goodrich says. "It's big. It's doable, but it's going to be tedious."

Doctors have placed what's called tissue expanders into the boys' heads. The expanders gradually get filled with fluid ahead of the surgery to stretch their skin.

Brain mapping like this, Goodrich explains, is nothing short of phenomenal. "This is all unique to this surgery," he says.

He's already spent countless hours studying the anatomy of the boys' brains. The imaging eliminates the guesswork and surprises that have popped up in prior surgeries.

"When you're doing a reconstruction like this, even when you're working on cranial-facial reconstruction, there's a lot of guessing," Goodrich says. With the new technology, he says, "you're no longer guessing."

The four tissue expanders placed in the boys' heads were first mapped to specific locations with the 3-D imagery. That was crucial in allowing the surgeons to know they'd have enough skin for reconstruction and to place the expanders away from incisions from the first three surgeries to lessen the chance of infection.

"None of that was ever possible before," Tepper says. "That's the first time that's ever been used."

Only 58 craniopagus separation surgeries have been reported worldwide since 1952.

Goodrich was thrust into fame in 2004 when he operated on Carl and Clarence Aguirre, twins from the Philippines who shared about 8 centimeters of brain tissue. The surgery was initially celebrated as the first craniopagus surgery in which neither child had long-term neurological damage. Carl, however, suffered neurological decline over the year and a half after the surgery, resulting in permanent disabilities, including a seizure disorder. He speaks only a few words and is confined to a wheelchair. Both boys still wear protective helmets.

Arlene Aguirre waits with her twin sons Clarence, left, 14, and Carl, right, 14, at Dr. James T. Goodrich's office at The Children's Hospital at Montefiore. Dr. Goodrich separated the conjoined twins in 2004.

Arlene Aguirre recently celebrated her boys' 14th birthday. Goodrich was there, as he's been for every birthday the past 12 years. "He never forgets," she says.

She feels the pain of the McDonalds and has spoken with Nicole, giving her advice and support. "I don't have any regrets that I did the surgery. Dr Goodrich did the best he could."

The surgeon adds, "the fact we were able to get one out with good functionality and the other out alive and doing well, but clearly with cognitive and motor skill delays," is still something to celebrate. The historical norm was that both children died during surgery or that one survived but with neurological devastation.

Over the years, Goodrich has learned to choose twins carefully; sometimes, their brain anatomy is so intertwined, the risks of surgery are too great.

On Thursday, Goodrich will skip breakfast and enjoy a cup of coffee, preferably a cappuccino, before beginning the surgery. His mantra is to "take it easy and slowly and carefully."

What's it like to separate twins?

"It's actually pretty awesome," he says. "It's chaotic. You're trying to get two kids on two tables. ... The problem is when you have nothing holding the brain in, the brain can actually slip and fall out. So, it's quite a treat to get them repositioned.

"We've got it down to a fine art, but in the beginning, it was a bit of a challenge."

Like Nicole and Christian, Goodrich's biggest concern is for Anias, because of the issues he's already faced. Traditionally, the weakest child is most at risk of dying on the operating table.

'God gave you these babies. They're meant for you'

Prior to all this, Christian and Nicole McDonald lived an uneventful life in Braidwood, Illinois, a town of 5,000 about 70 miles south of Chicago. He was a truck driver; she was a pediatric physical therapist.

Nicole plays with Jadon, the one she describes as the troublemaker but still "sweet and cuddly and he likes to be snuggled." She longs to hold each child individually.

They were busy raising their firstborn, Aza, when Nicole got pregnant and they went for a routine ultrasound in May 2015. It took just 30 seconds for the technician to blurt out, "Oh, you have two babies in there." Excitement immediately filled Nicole: She thought about the need for a bigger house, a van. They didn't have two car seats; they'd need to buy another.

Nicole returned home, intently focused on the October due date, when the phone rang. "You need to come back immediately," the voice at the other end of the line said.

Nicole responded that her husband was at work, and she wasn't sure whether she could get back. "No, you need to come now," the voice barked.

Her grandmother offered to watch Aza, and her mom, Chris Grasso, escorted her to the doctor's office. Again, Nicole was told no relatives could join her in the examination room.

The second ultrasound took about two seconds. The radiologist stepped into the room to deliver the news. "You know, Mrs. McDonald, I don't know what to tell you, but your twins are conjoined," the woman said.

"What do you mean conjoined?'" Nicole asked.

"From what it appears, they share a head. We don't know how much."

Her memory of that day gets fuzzy after that.

Nicole was told she was being referred to the maternal fetal specialty unit at Rush University Medical Center in Chicago. "We'll get you an appointment. I'm sorry," the radiologist said.

In a daze, Nicole stumbled into the waiting room and burst into tears. She told her mother to get the car.

Inside the vehicle's confines, Nicole continued to sob. "What's wrong? What's wrong?" her mother asked.

"These babies are conjoined," Nicole said.

"What does that mean?" her mother asked.

"They're conjoined at the head. I don't know what we're going to do."

Her mom responded with a quiet stoicism: "God gave you these babies. They're meant for you."

Her mother's words were a game-changer for Nicole. The tears soon stopped flowing. She immediately went into research mode -- determined to give birth to the babies, no matter what.

But first, she had to tell her husband. She hesitated to call him. She worried that he would be driving his truck and might crash. When she dialed his number, he picked up immediately. He was sitting in a parking lot.

Christian cried at the news. He was afraid his wife could die during birth, and he worried for the health of their twins.

Even just turning over the twins is an ordeal. It takes both parents acting in using to flip them over. They must then make sure both can breathe properly.

"Don't worry. We'll figure it out," Nicole told him. "I've already been on the computer." She kept a journal, including every doctor and medical department that could be of help. When she called Boston Children's hospital, recommended as one of the best programs in the country, they called her back within five minutes. They also recommended the gray-bearded surfer, Dr. James Goodrich.

Nicole says that at every doctor's appointment up to 20 weeks, they were asked whether they wanted to abort the pregnancy. "Terminate" is the term the doctors always used.

Friends and family suggested it, too; an abortion would be more merciful, they said. "I even had to think about it," Christian says, "because I didn't want to make some rash decision."

Ultimately, they knew they would take this journey together, as a family. "I heard their heartbeats," Nicole says.

The boys were born nearly a month early, on September 9, 2015, via an unscheduled C-section. Arrival time: 11:11 p.m. "They were normal little boys, like any other two little babies you would see -- except for being conjoined," Christian says.

They planned to name them Caiden (for "warrior") and Tavish (for "twin"). But Dad flipped through the book "100,000 Baby Names" at the last minute and stumbled across the name Anias, also of Hebrew origin. He marveled at its definition: "God answers."

It was the perfect match for another name they'd liked for its meaning: Jadon, "God has heard."

As he looked at his boys in the neonatal intensive care unit, Christian felt that God had spoken.

The boys had their names.

A new life for the family, in preparation of new lives for the twins

Much has happened over the past year. Mom and Dad quit their jobs and moved from their tiny Illinois town to the heart of the Bronx to be close to the hospital. Continuing to work a full-time job was not possible: The twins consume their entire day, even with nurses helping out 16 hours a day.

The parents must also tend to their 3-year-old, to make sure he's not lost amid the commotion. Aza has grown quiet; he's been known to slip outside, climb the fence and sneak onto a neighbor's trampoline.

Even a simple family outing can be stressful. The twins must be placed on the floor of their minivan, and that requires Mom and Dad to lift them in unison and walk together.

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An online fund has been started to help the McDonald family with their medical and living expenses. Click here if you'd like to contribute.

The McDonalds live solely on the generosity of others. The family receives packages in the mail with clothes for the boys. Strangers send checks, too. One donor who has been a generous contributor to the hospital is helping cover living expenses.

The cost of the surgery is about $2.5 million. The parents have health insurance covering much of the expense, but not all. The family also has set up a GoFundMe page to help offset the cost.

An 83-year-old neighbor has taken particular interest in the family. Martin O'Malley is an Irish immigrant who has lived on the block for 45 years. He helped the McDonalds get meals delivered to the home when they first arrived. Neighbors and strangers drop off money at O'Malley's house to take to the family. A local mechanic told him he wouldn't charge for fixing his car: "You give the money to that family."

"I love them -- all of them. The little twins get to me," he says, fighting back tears. "I'm a twin myself. My other half, she died at birth."

He looks up to the sky and holds his hands up: "Please God, may it all work out."

Preparing for the big day

On a loveseat in the family living room, the McDonalds say their joy overwhelms their doubts surrounding the surgery.

"We just feel good about the outcome," Christian says.

"I'm prepared mentally and emotionally," Nicole says. "It's just the preparation for that day that is hard."

Like seeing the boys scream every time the tissue expanders are filled with more fluid and being helpless to console them. At one recent appointment, Jadon flashed an angry look at Dr. Tepper, as if he were saying, "Stop right now!"

"I had never seen a 1-year-old give a look like that," Nicole says.

The first 72 hours after the operation will be the most crucial and life-threatening. If the boys do well through this time, they will probably spend the next several weeks in the pediatric critical care unit. The next stop: an inpatient recovery rehabilitation center, possibly for several months, where they will continue to heal and learn to roll over, lift their heads and achieve other infant milestones. They will be outfitted with protective helmets.

The surgeon is not worried about the boys' speech being affected because of the area of the brain where he's operating. It's their motor skills that will need the most rehabilitation. "They have no back control because they've never sat up," Goodrich says. "They've never crawled."

Nicole longs to hold each twin in her arms. She's not sure when that moment will arrive, but she's pictured it many times. "There's nothing harder than watching your child cry and not being able to pick them up," she says. "To hear them cry and react to it in a motherly way is something I'm really excited for."

The entire surgical team shares her feeling. Tepper, the plastic surgeon, became a father right around the time he met the twins. "There's a different perspective when you become a parent," he says, adding that he can better empathize with the McDonalds and "how much stress and anxiety they must be feeling through this process."

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"It's going to be exciting to separate them," he adds, but the ultimate satisfaction will come when doctors can hand "these individual boys to their parents separately."

It will be a simple and powerful moment -- the kind the McDonalds say they've learned to appreciate. Going to the park. Taking a stroll with your kids. Hugging them.

That's their advice to others -- to cherish life's small moments -- even as they approach their biggest.