Positional plagiocephaly is a flattening of one side of a baby’s skull caused by external pressure, not by any problem with the skull bones themselves. It’s one of the most common infant head shape concerns, affecting roughly 6% to 15% of babies in the first year of life, with higher rates in premature infants (about 12%) compared to full-term babies (about 5%). The flattening typically appears in the weeks after birth and gives the head an asymmetric, slanted look when viewed from above.
What Causes the Flat Spot
A newborn’s skull is soft and made up of separate bony plates that haven’t yet fused together. This flexibility is essential for passing through the birth canal and for accommodating the brain’s rapid growth in the first year. But it also means the skull is vulnerable to being reshaped by sustained pressure.
The single biggest contributor is sleeping on the back. Babies placed in the supine position have about 2.7 times the odds of developing positional plagiocephaly compared with babies who aren’t. This is a well-known tradeoff: back sleeping dramatically reduces the risk of sudden infant death syndrome, and that benefit far outweighs the cosmetic concern of a flat spot. But it does mean babies spend many hours with the same part of their skull pressed against a firm surface.
A head-turning preference makes things worse. Most babies naturally favor turning their head to one side, and that preference is the strongest individual risk factor. A rightward preference carries about 4.7 times the odds and a leftward preference about 4.2 times the odds of developing flattening. This often traces back to positioning in the womb: most babies settle into the birth canal with their right side of the head pressed against the mother’s pelvis, which can set up a postnatal comfort habit of turning the same direction. Boys are at slightly higher risk, likely because they tend to have larger, less flexible heads. Vacuum or forceps-assisted delivery also modestly increases the odds.
How Torticollis Plays a Role
Torticollis is a tightness or shortening of the muscles on one side of the neck that limits how far a baby can turn their head. It’s one of the most important risk factors for positional plagiocephaly because it locks the baby into resting on the same spot night after night. If torticollis persists beyond six months, the risk of a more severe flat spot increases significantly.
Physical therapy is the standard approach. Gentle passive stretching and exercises to strengthen the neck and upper body help babies regain full range of motion. One study found that hands-on pediatric manual therapy improved head rotation by nearly 30 degrees on average, compared to about 6 degrees with a home exercise program alone. Addressing the neck tightness early is one of the most effective ways to prevent worsening skull asymmetry.
What the Skull Looks Like
When viewed from above, a baby with positional plagiocephaly has a head shaped like a parallelogram. One side of the back of the skull is flat, and the forehead on the same side may bulge forward slightly. The ear on the flat side often shifts forward compared to the other ear. There may also be a compensating bulge at the back of the skull on the opposite side. These features create a diagonal asymmetry that doctors can measure by comparing the distance across the skull along two diagonal lines. A difference of 9 to 12 millimeters between those diagonals indicates mild to moderate asymmetry, while anything over 12 millimeters is considered severe.
How It Differs From Craniosynostosis
The key distinction is that positional plagiocephaly involves a normal skull with open, flexible joints between the bone plates. Craniosynostosis, by contrast, happens when one of those joints fuses too early, physically restricting how the skull can grow. The two conditions produce different head shapes. Positional plagiocephaly creates a parallelogram shape, while premature fusion of the lambdoid suture (the joint at the back of the skull) produces more of a trapezoid shape. In craniosynostosis, you can often feel a bony ridge along the fused joint, and a bulge may appear behind the ear on the affected side.
Timing also helps distinguish them. Craniosynostosis is typically present at birth, while positional flattening usually develops over the first several weeks of life. In uncertain cases, a CT scan can confirm whether the skull joints are open and normal.
Why the First Year Matters Most
About 85% of a child’s total skull growth happens in the first 12 months. The fastest period is the first six to eight months, when the skull can grow nearly 20 millimeters in length and 10 millimeters in width. After the first birthday, growth slows considerably and the bones begin to harden and lock together. The soft spot at the top of the head typically closes between 14 and 22 months, and by 9 months of age, over 90% of the connective tissue between skull plates has already turned to bone.
This timeline is what makes early intervention so important and also why positional plagiocephaly tends to self-correct to some degree. Once a baby starts sitting up, crawling, and spending less time on their back, the pressure is removed and normal growth can partially reshape the skull.
Treatment Options
Most mild cases improve with repositioning strategies alone. The goal is to reduce how much time the baby spends with pressure on the flat spot. Practical steps include alternating which end of the crib the baby’s head is placed at, changing the direction you hold the baby during feeding, and building in plenty of supervised tummy time. The NIH recommends starting with two or three short sessions of 3 to 5 minutes each day, working up to 15 to 30 minutes of total daily tummy time by about 2 months of age. Tummy time strengthens neck muscles and naturally reduces the hours spent pressing on the back of the skull.
For moderate to severe cases that don’t respond to repositioning, a cranial remolding helmet (also called a cranial orthosis) may be recommended. The helmet works by leaving space over the flat area so the skull can grow into it, while applying gentle contact on the prominent areas to redirect growth. Treatment works best when started between 5 and 6 months of age, though it can still be effective up to about 18 months. All babies who achieved successful correction in one study wore the helmet for more than 17 hours per day, and wearing it over 20 hours per day shortened overall treatment time. The typical course of treatment lasts several months.
Long-Term Effects on Development
Whether positional plagiocephaly affects brain development is one of the most studied and debated questions in pediatric medicine. A prospective study that followed children into school age found that kids who had moderate to severe flattening as infants scored lower on tests of reasoning and academic skills compared to unaffected children. The differences were meaningful but not dramatic, roughly in the small-to-moderate range across measures of verbal ability, nonverbal reasoning, and reading and math performance.
Children who had only mild flattening, however, showed little to no difference from their peers on the same tests. And the researchers were careful to note that the association does not prove the flat spot itself caused the lower scores. Positional plagiocephaly may simply be a marker that travels alongside other developmental risk factors, such as limited movement variety in infancy or underlying differences in muscle tone. The flat spot could be a signal rather than a cause, which is why pediatricians monitor developmental milestones in these children rather than assuming the skull shape alone is the problem.