What’s With That Soft Spot On My Baby’s Head?
“It’s easy,’ she says. “You guys all have the same little habit of poking at the top of a baby’s head.”
She’s right. Force of habit I guess, but every time I meet a new baby, whether it’s in the supermarket or my office, I always want to put my finger on the soft spot at the top of their head. You know the one I mean. The spot in the midline just behind the hairline (or future hairline; some of these little tykes don’t have much for hair).
It’s called the anterior fontanelle. Every parent knows it. Some parents fear it, perhaps afraid they might damage something inside if they push too hard (they won’t). Others fixate on how quickly it is or isn’t closing, which—with very few exceptions—matters not at all.
So what is that spot all about anyway? Why is there a hole in my baby’s skull?
Actually, there’s lots of holes in your baby’s skull, this is just the most obvious. There is a much smaller posterior fontanelle, which generally closes very early, within a month or two of birth (which is probably why few parents even know it’s there). There are also tiny fontanelles on the side of the head in several cases, which close even faster. Other openings allow for the passage of nerves (like the optic nerves for the eyes—which actually aren’t nerves at all but an extension of the brain and thus the only part of the brain actually visible externally in a normal person). There’s a big opening for the spinal cord at the base of the skull, called the foramen magnum. And many openings for smaller nerves and blood vessels.
But what about the anterior fontanelle? Nothing passes through that, right? Right.
The fontanelles are membranous patches, which, as we all know, close eventually (if you don’t know and are old enough to read this, just reach up and try to find your anterior fontanelle now—you can’t, there is no remnant after it closes). Usually the anterior fontanelle closes by a year of age, but it can close as early as six months or as late as eighteen months and still be normal.
It is, of course, a remnant of the newborn’s need to pass through the birth canal. Though you might not want to think about it, a newborn’s head is too big to pass through that little tunnel of love. My wife, who has given birth four times, once likened it to passing a watermelon out of your ass. Or perhaps she was suggesting I try that, I can’t quite remember. Oh well. My wife is a smart woman and who am I to argue with her?
Except, in this case, the good Lord has provided. The cranial vault, otherwise known as the brain case, is comprised of several bones: the paired frontal, parietal, and temporal bones, as well as the two unpaired bones, the occipital (at the back of the head—which contains the biggest hole in the skull, the large foramen magnum noted above) and the oddly named and even more oddly shaped sphenoid bone, with its greater and lesser wings making it look something like a bat. It turns out the sphenoid bone has an extremely complicated 3-D shape and is of great importance to the neurosurgeon because it’s various wings (and the various edges and ridges associated with them) are excellent landmarks to guide us as we move about during brain surgery.
In addition to enclosing the seat of human intellect (or lack thereof in some folks), these bones have one other commonality—they can articulate and slide over one another during birth, only to become rigid in the months that follow until the idea they could ever have been flexible in the first place is unthinkable.
Have you ever looked at a newborn in the first hours and days after birth and imagined them to be coneheaded, like on the old Saturday Night Live skits? Well, you weren’t imagining that of course. During the passage through the birth canal, the bones of the cranial vault overlap just enough to allow the head to distort as it slides though a woman’s pelvis. They can do this because the joints between the bones, the sutures, are membranous and distensible.
The anterior fontanelle is simply the confluence, that is the intersection, of four of these membranous sutures:
1. The suture between the paired parietal bones—the sagittal suture, which runs along the midline of the head from front (the anterior fontanelle) to back (the posterior fontanelle);
2. The suture between the parietal bone and the frontal bone on each side of the head—called the coronal suture; there are thus two coronal sutures, one right, one left;
3. The suture which splits the frontal bone in the midline—the metopic suture. The frontal bone, a large single bone in the adult, is actually two bones in the infant; the metopic suture is the continuation of the sagittal suture from the anterior fontanelle to bridge of the nose and occupies the real estate in the exact center of the forehead.
So, there you have it. The anterior fontanelle is a remnant of our passage through the birth canal and really serves no useful function after that.
Except to the inquisitive finger of the neurosurgeon.
Turns out, you can tell a lot about a baby by the state of their anterior fontanelle at rest. That, however, is a subject for another day.