Like any new parents, Heather and Riley Delaney are excited to see their babies hitting developmental milestones.

But for 15-month-old twins Abby and Erin, who were fused at the tops of their heads until June, even the tiniest hints of progress win tears and applause.

"Look at Abby holding her head up!" occupational therapist Ashley Binkowski said recently as she helped the baby sit on a mat at Children's Hospital of Philadelphia. "That takes abdominal control on this soft surface."

"Working on that core!" her father cheered.

Heather, who has not been home to Mooresville, N.C., since she was 26 weeks pregnant, is keenly aware that the road ahead may be tough, especially for Abby, who lost part of a vital cranial blood vessel she once shared with her sister. But if the hurdles are high, so are the hopes.

"Babies have brain plasticity, which means their brains can overcome a lot," Heather said. "The doctors have a lot of hope for what the girls can do. But we won't really know what kind of deficits they have until they're about 3. For now, they're doing fantastic."

So much so that, with luck, the entire Delaney family will celebrate Thanksgiving at home in North Carolina. There, the twins will resume intensive rehabilitative therapies.

"What parents really want to know is that crystal-ball question about the future outlook," said CHOP pediatrician J. Michael King, a rehabilitation medicine specialist. "But what we look at is functioning. Both girls are making gains. They're engaging, and looking around, and knowing faces, and babbling. I have high hopes for them."

Joy and terror

When CHOP disclosed the separation surgery a week after it happened in June, a news release offered only basic details: Heather consulted the hospital and began biweekly checkups soon after an ultrasound at 11 weeks revealed the conjoined twins. She moved into CHOP's special maternity unit at 27 weeks so she could be closely monitored. The babies were born by cesarean section on July 24, 2016, about 10 weeks premature and weighing 2 pounds each. Eleven months later, they were separated by a medical team of 30 in a marathon operation.

Heather's heartfelt blog — which she started after that shattering ultrasound and suspended just after the twins were born — personalized those basics. Early last month, when the couple sat down for an interview and let an Inquirer and Daily News reporter observe the twins' therapy, a fuller portrait emerged of the parents' devotion to their babies, to each other, and to God.

Heather, 27, and Riley, 24, started dating when they were teen youth leaders at their church in Port Jervis, N.Y., where they grew up. She calls him "the love of my life" on her blog, though at first she rebuffed his romantic overtures.

"She was my best friend, but she thought I was too young," Riley recalled.

She went on to graduate from Valley Forge Christian College — now the University of Valley Forge — in Phoenixville and worked as a nanny. Riley got into construction and renovates shops for an e-cigarette company.

Three months after they married in October 2015, they were delighted to discover she was pregnant.

Then they were whipsawed by the ultrasound that showed a happy surprise – twins — and an unimaginable complication.

"Something that was 100 percent joy turned to 100 percent terror," Heather blogged.

Her obstetrician referred her to CHOP, which had separated 23 sets of conjoined twins over the last 60 years. Even CHOP, however, had never operated on a pair fused at the head, called craniopagus twins.

"We didn't really figure anything out. God just paved the way for us," Riley said. "We didn't know how we were going to pay our rent, much less pay for a plane ticket. Then someone would donate it."

With Heather in Philadelphia, Riley moved into her parents' garage to save money. They also created a GoFundMe page, which has raised $25,000 toward a goal of $100,000.

Riley, who has been trying to find a fixer-upper house that they can afford, relieves his anxiety by "working a lot."

"We're blessed to be their parents," he said. "It's stressful — but the girls aren't stressful. It's being separated and having to travel."

Heather frowned. About the closest she gets to stress relief is comparing notes with mothers of sick children at the Philadelphia Ronald McDonald House, where she has been living. Riley immediately sensed her mood shift.

"I'm sorry," he said. "Did you want to say something?"

"It's a little different for me," she said softly. "I have definitely had my moments of, like, 'How did all this happen, and how am I here?' It's all worth it in the end, but it has not been easy. It has been so hard. No one ever wants to watch their kids go through something like this."

Heather's steadiness has impressed the twins' platoon of doctors, therapists, social workers, and nutritionists.

"I cannot say enough nice things about Heather," said reconstructive surgeon Jesse Taylor, who co-led the twins' separation. "She is so dedicated. She's an amazing woman."

Play is the twins’ job

The separation surgery, while successful, set back the girls' developmental clocks. While joined, they had begun to display typical 10-month-old behavior. They smiled, laughed, played with toys, and bounced in their crib, although maneuvers such as sitting and crawling were impossible.

Erin has always been the stronger, more advanced twin because she had a larger share of the sagittal sinus, a major cranial blood vessel that drains blood from the brain to the heart and other organs. Even when craniopagus twins have little or no brain tissue in common — the best-case scenario that made Abby and Erin candidates for separation — cutting their shared cranial blood supply can result in neurological damage, or death.

The twins' prematurity is also an issue.  Although each now weighs about 20 pounds, which is within normal at their age,  they are fed through tubes implanted in their stomachs to compensate for immature sucking, swallowing, and digestive reflexes.

"Erin can eat purees and smoothies, but she prefers not to," Heather said. "She just spits it out."

To help the twins catch up developmentally, they spend hours each weekday in physical, occupational, and speech therapies.

"Play is most important," explained therapist Binkowski, referring to the toys and gymnastic equipment that surrounded them. "Play is their occupation."

But it is demanding, exhausting, and not always fun for the twins.

During the session last month, Abby was lauded as the morning's "rock star." She stayed alert for the whole hour, propped herself up on her arms while lying on her belly, and smiled at her sister.

Erin, in contrast, was battling that typical toddler tribulation, a cold. As the therapist bounced the baby on a giant red ball, her hacking cough trailed off into a howl of complaint, reminding everyone that monumental surgery is no inoculation against routine childhood miseries.

Million-dollar babies

Erin's cold spread to Abby, then to Heather. No sooner was Abby better than she developed a mysterious blood infection that put her back in intensive care.

"She spiked a fever of 105," Heather said. "She had lots of spinal taps and every type of test to figure out where it came from and make sure it wasn't in her brain. Lo and behold, it was just a random infection, maybe from her sinuses. The good thing is, we had a chance to get new scans. Her brain is healing nicely. "

Still, Abby's progress was disrupted. She continues to need hospital care, while Erin — now staying in Heather's room at the Ronald McDonald House  —  can stand up with help and, to her father's delight, has added "Da-da" to her jabbering.

"Trying to split myself equally is tough because Erin does require a lot now; I do all of her care," Heather said a few days ago. "I have to make sure Abby gets as much attention. As soon as we walk in her room, a huge smile comes across her face and her little legs kick.  But she never gets fussy or seems to be anxious without us there. To her, it's normal. Abby is just a super easy-going baby."

In coming years, the twins will need reconstructive surgery to replace missing skull bone at the tops of their heads, now covered with temporary patches. Remarkably, they have not suffered any wound infections or scar ruptures.

CHOP social workers are helping the Delaneys arrange therapies back in North Carolina, and sort out state Medicaid insurance coverage, which was contested by their home state because of Heather's extended stay in Pennsylvania.

"I haven't seen the medical bills, but I know it's in the millions," Heather said. "It's thousands of dollars a day just for [intensive care]. They're definitely million-dollar babies."

What is her greatest fear as she looks ahead?

"Everything," she said with a mirthless laugh. "I just want them to be happy. That's all I want, is for them to live happy lives and be as normal as they can be. So anything that could hinder their happiness worries me.

"But they're doing so well that I have a lot of hope for the future."