Flat head in babies is almost always caused by repeated pressure on one spot of the skull. Because infant skull bones are soft and separated by flexible tissue, they can change shape when a baby rests in the same position for extended periods. The medical term is positional plagiocephaly, and it became far more common after 1992, when pediatricians began recommending that babies sleep on their backs to reduce the risk of SIDS.
The good news: this type of flattening is a cosmetic issue, not a medical one. There is no credible evidence that positional flattening affects brain development or causes any other health problem. Still, understanding the causes helps you prevent it or catch it early when it’s easiest to correct.
Why Baby Skulls Are Vulnerable to Flattening
A newborn’s skull isn’t a single solid bone. It’s made up of several plates separated by gaps called sutures, filled with flexible connective tissue. These gaps exist so the skull can compress during birth and expand as the brain grows rapidly in the first year. The skull bones typically don’t begin fusing together until around age two.
This flexibility is essential for healthy brain growth, but it also means the skull is moldable. Any sustained, uneven pressure on one area can gradually push it flat. The younger the baby, the softer the skull, and the more susceptible it is to reshaping.
Back Sleeping and Positional Preference
The single biggest contributor is time spent lying on the back. Babies sleep for many hours each day, and if they consistently turn their head to the same side, the same spot presses against the mattress over and over. That repeated contact flattens the area. Some babies develop a strong preference for turning one direction, which concentrates the pressure even further.
This isn’t a reason to stop back sleeping. The back-to-sleep guidelines dramatically reduced SIDS deaths and remain the safest sleep position. But the trade-off has been a sharp increase in positional flattening, which is manageable and, in most cases, temporary.
Tight Neck Muscles (Torticollis)
Some babies are born with a condition called torticollis, where a tight or shortened muscle on one side of the neck makes it difficult to turn the head freely. A baby with torticollis will naturally hold their head tilted or rotated to one side, which pins the same part of the skull against whatever surface they’re lying on. Torticollis and flat head frequently appear together for this reason.
If you notice your baby strongly favoring one direction when looking around or seeming unable to turn their head the other way, that neck tightness may be driving the flattening. Physical therapy to stretch and strengthen the neck muscles often resolves both the torticollis and the head shape issue together.
Time in Car Seats, Swings, and Bouncers
It’s not just the crib. Any device with a firm, curved surface that cradles the back of the head adds to the total pressure time. Car seats, infant swings, bouncers, and carriers all restrict head movement and keep the skull pressed against the same spot. Using them for travel or short breaks is fine, but extended awake time in these devices adds up quickly and limits a baby’s ability to move freely and shift positions.
Premature Birth and Multiples
Premature babies face a higher risk for two reasons. Their skull bones are even softer and less developed than those of full-term newborns, making them more moldable. They also tend to spend more time lying in a fixed position in the hospital, often on respirators that keep their heads still.
Babies from twin or triplet pregnancies are also more likely to be born with some degree of flattening already present. Crowded conditions in the uterus can put sustained pressure on the skull before birth, especially in the final weeks. First-born children and males also show slightly higher rates, though researchers aren’t entirely sure why.
Factors That Raise the Risk
- Low activity level. Babies who are less active or have lower muscle tone move their heads less often, keeping pressure concentrated on one area.
- Lack of tummy time. Without regular time on their stomachs while awake, babies spend nearly all their hours with the back of the head against a surface.
- Assisted delivery. Births involving forceps or vacuum extraction can temporarily affect head shape, and the initial asymmetry may make the baby favor resting on the already-flattened side.
- Multiple births or cramped uterine space. Pressure before birth gives the flattening a head start.
Craniosynostosis: A Different Cause
In rare cases, an unusual head shape isn’t from external pressure at all. Craniosynostosis is a congenital condition where one or more skull sutures fuse too early, forcing the skull to grow in an abnormal pattern. Unlike positional flattening, craniosynostosis is usually present at birth, produces a different and more pronounced shape change, and can potentially increase pressure inside the skull or affect cognitive development.
The two conditions look different on examination. Positional flattening typically affects the back of the head and creates a parallelogram shape when viewed from above, with one side shifted slightly forward. Craniosynostosis creates more rigid, defined ridges along the fused suture line. If there’s any question about which is causing the shape change, a pediatrician can usually distinguish them through a physical exam, sometimes with imaging. Children with craniosynostosis are referred to a surgical specialist.
How to Prevent Flattening
Tummy time is the most effective prevention tool. The National Institutes of Health recommends two or three short sessions per day, each lasting three to five minutes, starting from the first days at home. By around two months, the goal is 15 to 30 minutes of total tummy time spread throughout the day. This takes pressure off the back of the skull, strengthens neck and shoulder muscles, and encourages the baby to develop the ability to turn and reposition their head independently.
Beyond tummy time, alternating which end of the crib you place your baby’s head encourages them to turn in different directions to look at the room. When holding or feeding your baby, switch sides regularly. And while your baby is awake, limit time in car seats, swings, and bouncers to when they’re actually needed for travel or safety.
What Happens if Flattening Develops
Most mild cases improve on their own as babies gain head control, start rolling over, and spend less time lying flat. The combination of repositioning strategies and increased tummy time resolves the majority of cases without any further intervention.
For moderate to severe flattening that doesn’t respond to repositioning and physical therapy, a cranial molding helmet may be recommended. These lightweight helmets work by leaving space where the skull needs to grow while gently guiding the shape. Earlier treatment tends to produce better results, since the skull is most responsive while it’s still growing rapidly. Most helmet treatment happens before a baby’s first birthday, when the skull is still soft enough to reshape effectively. After the sutures begin closing around age two, the window for passive correction narrows significantly.