My son’s first haircut was by a neurosurgeon. If you want to get technical, it was probably a resident, not the surgeon himself, who shaved the rounded W that stretched nearly ear to ear across William’s scalp. But that wouldn’t sound as quippy on the onesies sold in the online support groups.

William was 5 months old when I watched him being rolled down a hallway on the seventh floor of the Ann & Robert H. Lurie Children’s Hospital to get that haircut. He lay in the center of a child-size gurney, wearing a diaper and a miniature polyester hospital gown the color of butter, his arms and legs gently curling in the air in that way babies perpetually move. He seemed perfectly healthy. And yet my husband, Alex, and I had no choice but to place our son in the hands of a doctor who was going to remove a portion of his skull to fix a problem that wasn’t dangerous yet—but had a chance of becoming so if we just let it be.

Three weeks earlier, we’d taken William for his four-month checkup at our pediatrician’s office in Oakbrook Terrace. His weight was on track—still light for his age, but following a consistent upward curve. His height was in the middle percentile. But his head (always large) had, by the nurse’s measurement, jumped past the far reaches of the chart. The pediatrician, unfazed, took out a tape measure and wrapped it around his skull herself. She clucked decisively: The nurse had measured wrong. William’s head was large—97th percentile—but not off the charts. And its growth was steady. No sudden swelling or stalling. But that blip in the checkup prompted Alex to ask a question.

“What do you think about the shape of his skull?”

Everybody knows that newborns have funny-looking heads, compressed into all manner of oddball forms during delivery. William was no longer a newborn, but his head still seemed unusual. From above, it was an oval. Long and narrow. Flat on the sides. His forehead was high and bulged forward slightly. Covered in short tufts of downy strawberry blond hair, though, it all looked sweet.

Our pediatrician agreed that William’s head looked a little flat. She felt his front soft spot and said that it was small—about the size of a fingertip—but still open, as it should be. There wasn’t anything to worry about. She explained that lying down for extended periods of time on one side or the other had probably molded the cranium into a shape that was peculiar, yes, but would correct itself the more he sat upright. If it didn’t by his six-month appointment, we’d look into a helmet to coax his oblong dome back into a sphere.

Flat head. They’d taught us about it in baby care class. It’s what happens when you don’t do enough tummy time or leave your child too long in swings and cradles, those crutches of new parents.

My cheeks prickled. I hadn’t done enough tummy time, at least not as much as the books and blogs recommended. Every time I tried, William bleated like a despondent lamb. I just couldn’t bring myself to put him through it for more than a few minutes a day. But he never lacked for time spent supine in my arms or cradled against my shoulder. He simply wouldn’t nap anywhere else.

As the pediatrician wrapped up the exam, she gazed appraisingly at William’s head and asked, “Has he always looked like this?”

Yes, he had, so far as I could recall from those bleary early days.

I will never forget the look that flashed across the doctor’s face on hearing my response. It didn’t even last a second, her eyes registering a flicker of alarm before she quickly collected herself. The soft spot was still open, she said. Everything was fine.

Like most new moms, I experienced a delirious mix of terror and delight during the first hazy postpartum weeks while my body put itself back together and I got to know this mysterious little person. William was a dreamy newborn, who offered his first milk-drunk smile at only two and a half weeks. I spent my maternity leave studying every molecule of his body: the dimples at the base of his fingers, eyebrows so fine they appeared transparent, lips that beckoned like a baby bird’s beak when he was hungry. Of course I noticed his atypical head. It just never struck me that this was a problem. William was healthy and cute.

And yet, despite the pediatrician’s confidence, the exchange we’d had with her at the end of our visit nagged at me. William’s head had always looked this way. And that fact had—however fleetingly, just long enough to immobilize her gaze for an infinitesimal moment—given the doctor pause.

That night I nursed William to sleep and then pulled out my iPad. I pecked away on Google: “Head flat on both sides.” “Elongated baby head.” “Long narrow baby head.” I was surprised that plagiocephaly—the medical term for the kind of flat head that can come from an infant lying around too much—wasn’t the only, or even the first, result that my searches turned up.

Medical illustration after medical illustration popped up on my screen, each one matching the outline of William’s skull. The images all showed a condition called scaphocephaly; the name, I read with a twinge of indignation, comes from the Greek for “boat head.”

Anxiety shot through me as my eyes darted from article to article. Scaphocephaly, I learned, doesn’t usually happen because of how you position your baby. In many instances, it’s the outward manifestation of a birth defect called sagittal craniosynostosis—a fusion of the top two plates in the skull. There are several types of craniosynostosis, each defined by which of the major plates have merged. But even as a collection of disorders, the condition is extremely rare, affecting roughly one in 2,500 babies. (By contrast, roughly one in 100 is born with a heart defect.)

In sagittal craniosynostosis, the fusion causes the brain to grow along the path of least resistance, pushing the infant’s still-malleable skull up and out in the front and back. The result is a head that appears charmingly off-kilter in a baby but can become a disfigurement for an adult. Worse, in rare cases, pressure on the brain can be devastating, leading to severe headaches, seizures, blindness, or cognitive and speech delays. Cranio, as the condition is casually called, almost always requires surgery. The pictures I stumbled upon were horrific: infant skulls completely exposed as they were refashioned from an egg into a globe.

I kept tapping the screen and reading: The telltale sign of sagittal craniosynostosis, beyond the head’s oblong shape, is a ridge that extends across the top, like a little mountain range, where the two plates have melded into one. I quietly padded over to William’s crib and traced my fingers over the top of his head.

There it was, a ridge so obvious I couldn’t believe I’d never felt it.

Everything is fine. I repeated the pediatrician’s words in my head like a mantra. I would call her back in the morning. Everything is fine. Everything is fine.

The following day, when I explained to the pediatrician’s nurse what I’d read the night before, she heaved a sigh—no doubt annoyed at another Google-happy mother who thought she knew better than the doctor—and said that as long as William still had a soft spot, even if it was small, he didn’t have a fusion.

Acting on my journalist’s instinct to research a topic to the point of obsession when I’m unsatisfied, I sought out other sources, worry creeping ever deeper into my heart, my head throbbing from sleeplessness and a case of sinusitis that had been lingering for weeks. When I mentioned my concerns about William’s head to our daycare provider, she nodded. “I noticed it the first day you brought him.” She urged me to get a second opinion. I emailed a friend who’s a pediatrician. It was probably flat head, she wrote back. And then, in the next sentence, there was that word again: craniosynostosis. It was a less likely explanation, she said, but a possible one. Her cousin had had it as a baby—discovered by Grandma, missed by the doctor.

The next day, I started an antibiotic to treat my sinus infection. That night as I nursed William, hives spread across half of his body in reaction to the traces of the drug that had come through in my milk. I called our pediatrician, who told me to give him Children’s Benadryl.

“While I have you on the phone,” I said, “could you tell me one more time what you think about his skull?”

The shape would correct on its own, she repeated. If it didn’t by six months, helmet.

But was there any chance he’d need surgery?

“Oh no,” she said. “There is no way he’ll need surgery. I promise.”

After I got off the phone, I gave William the Benadryl and rocked him to sleep. Still feeling less than reassured, I posted an overhead photo of him to the Facebook page of the moms’ group I’d joined. I was fishing for soothing responses, and they flooded in—how they wanted to kiss his little head, how adorable he looked. It all seemed like part of the continuum of normal.

Then a final comment appeared, from a mother I had met only digitally: “My son had craniosynostosis. He had surgical correction when he was 3 mos. He is 3.5 years old now, and is totally fine. Happy to talk more if you need to.”

My breath caught in my throat. I hadn’t mentioned anything about craniosynostosis in my post, and yet merely by looking at the photo, she’d come to that conclusion.

Hurried physicians often assume that patients’ symptoms correspond to the most common of plausible diagnoses—an example of a phenomenon known to psychologists as the representativeness heuristic. It’s a necessary shortcut when doctors are seeing dozens of patients a day, and the fact is, most of those patients do end up having mundane problems. The approach can prove devastating, however, when it causes doctors to overlook subtle clues that things are not what they may most conveniently seem.

On average, patients with unusual disorders receive two to three misdiagnoses before getting the right answer, according to Global Genes, an organization that advocates rare-disease research and awareness. Flat head is familiar. Craniosynostosis is not. And with only a perfunctory look, the two conditions can appear to be one and the same. The patient’s family often ends up realizing the difference before doctors do, forcing them into the role of advocate and sending them on a frustrating quest to find a physician who can corroborate their conclusions.

In this respect, Alex and I were luckier than most.

I’d known Tadanori Tomita, the chief of neurosurgery at Lurie, for more than a decade as the father-in-law of one of my best friends, Heather Ballard. When I joined the staff of Chicago magazine in 2014 and excitedly told Heather that Dr. Tomita had made our annual list of the area’s best doctors, the look she gave me amounted to “Duh.” (He is on this year’s list too, his 12th consecutive appearance.)

Hours after receiving the matter-of-fact Facebook comment about craniosynostosis, I texted Heather some photos of William’s head. I told her that I knew her father-in-law probably wouldn’t want to speculate based on cell phone snapshots, but if there was any chance he could at least take a look …

At 7:20 the next morning, Heather texted me back. “He said he could see you guys this afternoon in the Northbrook clinic if you have time.”

A few hours later, Alex, William, and I were in Dr. Tomita’s exam room. The doctor walked in, smiled, shook our hands, and began to gently touch William’s head.

Calmly and without the least hesitation, he said, “This is sagittal craniosynostosis.”

I started to cry.

“So you know what that is?” he said.

I already knew more than I wanted to.

Sometimes the defect is hereditary, Dr. Tomita explained. In such cases, parents will frequently have an aha moment: “Grandma had a football head too.” But often it’s just random bad luck: a gene that misfires during fetal development.

He then described a corrective procedure called an extensive craniectomy with recontouring. It entails removing a roughly four-by-four-inch section from the top of the skull, thereby creating a large soft spot that will give the baby’s brain the space it needs to grow. Then vertical incisions are made along the sides of the skull to reshape it into a more typical appearance. The greatest risk is blood loss. Around 80 percent of patients require a transfusion. The optimal window to perform the surgery is relatively narrow. After five or six months, the skull starts to lose malleability, making it harder to reshape and in some cases requiring a more invasive surgery to correct the deformity. Studies have also shown that age can be a tipping point for cognitive, speech, and behavioral delays. William’s window would close in six weeks.

If you have ever doubted the miraculous resilience of the human body, think about this: No bone graft or metal plates would protect William’s brain as he healed. Within three months, Dr. Tomita said, most of the excised section of skull would grow back along the proper path. William would look like an entirely different boy—the slope of his skull lower, the sides wider. No more boat head.

We learned a lot over the next couple of weeks as the surgery date approached. For one, my parents and I all share William’s blood type, which meant all three of us could donate in case he needed a transfusion. (He did.) Also, the number of homemade casseroles you receive during a medical crisis is directly proportional to its gravity. (We got seven.) And then this: Babies, in their innocent obliviousness, are very good at keeping you focused on the present, even when you’re terrified of the future.

Ten days before the surgery, we brought William back to his pediatrician for a preoperative physical. I didn’t want to return to her, but Dr. Tomita’s clear-thinking nurse pointed out that at least our doctor would have a consistent medical history going back to the first day of William’s life.

“You guys!” the pediatrician said when she saw us, throwing open her arms, bug-eyed. “What happened?” She never acknowledged her mistake, saying only that she had encountered cranio just two other times in her career. Rather than confront her or play along with the glib act, I stayed neutral so the focus would remain on William. But you can bet I grilled her on every single aspect of his fitness for surgery. Later, I asked a few doctor friends if they knew her reputation. (It had, in fact, been a doctor who recommended her to us in the first place.) They all gave us basically the same response: Patients love her. She’s very nice. They’d heard good things. No one thought she was a bad pediatrician. Many made excuses for how easy craniosynostosis can be to miss. The endorsements were scant comfort to Alex and me. We couldn’t help feeling that there must be something systemically wrong with the way family physicians approach situations like ours—failing to listen, really listen, even when parents repeatedly tell them something is wrong. It probably goes without saying, but she is not our pediatrician anymore.

We’ve heard from other parents, anecdotally, that doctors misdiagnose craniosynostosis as plagiocephaly more often these days, an unintended consequence of the Safe to Sleep campaign, which encourages parents to place newborns on their backs to reduce the risk of sudden infant death syndrome. The national effort has been a huge public health success—but in the estimation of some parents and doctors, more babies end up with flat spots. Because of that, and the prevalence of the representativeness heuristic, more babies with cranio presumably slip through the cracks.

At around 11 a.m. on June 12, a date that will forever be known as William’s cranioversary, our son entered the operating room. The surgery would take roughly two hours. Heather, a doctor at Lurie herself, brought Do-Rite doughnuts for me and Alex and the surgical team. She sat by our side while we waited, giving us updates as the anesthesiologist texted her from the operating room. When the procedure was over, Dr. Tomita beckoned us into the hallway. William’s surgery was a success. The top suture, or seam, of his skull had indeed been entirely fused. After removing that section, Dr. Tomita had remodeled William’s skull into a perfect vessel, giving his brain all the room it needed to grow. He would develop like an ordinary boy.

Everyone told me that babies are tougher than they look. But it still seemed like a small miracle when, less than 24 hours after the surgery, William’s swollen little face cracked its first smile. And it seemed like a huge miracle when, the next afternoon, he was already holding court with five nurses who were standing around his crib. He flirted, played, and kicked his chubby legs, made puffier from all the IV fluids. His heart rate spiked when we gave him his favorite toy. By the end of the third day, he was home.

Obviously, we knew William would look different. But the physical change was shocking. His eyes weren’t as puffy because, one doctor theorized, his brain no longer pressed against his sinuses. (He didn’t snore anymore, either.) Before, his tall, narrow forehead had made him look more like a boy than an infant. Now, with it rounded out, he looked like the baby he was always meant to be.

Among the mementos we have from his surgery is a small plastic bag that one of the nurses placed in a corner of his hospital crib. Inside is a tuft of William’s strawberry blond hair, saved for us by a surgical resident. The words “My First Haircut” are written on the bag, with a heart drawn just below.