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Do not get a big head, your mother might have told you. That's good advice, but it's too late for most of us. Relatively speaking, humans have had big heads for hundreds of thousands of years, much to the dismay of our mothers.

Our oversized noggins are causing great pain at birth. Babies have to turn and turn when leaving the birth canal, which sometimes leads to complications requiring surgery. And while big heads can be painful for the mother, they can literally transform themselves into babies: a pliable skull of a fetus deforms like a putty that is pushed through a tube so it can enter the world.

Intoxicating science

This skull deformity has been known for a long time, but in a new study, scientists from France and the US have observed how they used an MRI device during labor. The pictures, published in a study in Plus one, Showing how the skulls (and brains) of seven infants were squeezed and distorted during birth to pass through the birth canal. They also shed new light on how our skulls change in the course of our birth.

Researchers recruited pregnant women in France to undergo MRI a few weeks before pregnancy, and another in the minutes before they actually started giving birth. A total of seven women were scanned in the second phase of labor when the baby began to emerge from the uterus. They were then taken to the maternity ward to actually complete the birth.

The results are shocking, though not completely surprising. Skull deformity is a normal part of birth, a result of millions of years of evolution that has given us big brains and smaller pelvis. As expected, all seven normal babies have changed their heads at birth to conform to their mother's birth canal. However, this is only the second time that the process is in action, and the resulting images are impressive.

Five of the babies were born naturally and two by caesarean section. In five babies, the skulls quickly returned to their normal shape, while two remained deformed for an extended period of time. All but one baby were healthy, as measured by the postpartum APGAR test, which evaluated physical signs of breathing, color and muscle tone. The single baby, who scored less points in the test in the first minute after birth, gave a normal cut after 10 minutes.

Deformation of the brain

A reconstruction of a fetus before (above) and during (down) labor, showing how the brain changes shape. (Source: Ami et al./PLOS One)


The most notable finding for the researchers was how much each baby's skull changed when it passed through the birth canal. The skull of a baby is not a single bone like that of an adult, but consists of a few sections, which are connected by tough tissue to so-called "seams". These sutures allow the skull to change its shape when a baby is born and slides over and under one Another resembles the tectonic plates of the earth.

How much a skull changes depends on the shape of the mother's birth canal, the baby's head size, and probably the strength of the tissue that holds the sutures together. Some deformation is required for a baby to leave the birth canal, but too much can be a sign that something is wrong. The two babies born with caesarean section and the naturally born baby with initial health problems also showed the greatest deformity.

Birth skull deformation

An MRI of a fetus before (left) and during (right) labor shows how the skull deforms. (Source: Ami et al./PLOS One)

The skulls of most infants return to normal shortly after birth. However, the researchers found that two of the seven babies studied maintained the shape of a "sugarloaf" for some time, but without any noticeable health effects. And scientists add that understanding how far a baby's skull shifts during labor helps explain why newborns often have bleeding in their eyes and brain. This bleeding is usually not harmful to a baby, but it seems likely that the deformities that their skulls undergo are not without side effects.

This study was quite small, so the researchers find that more work has to be done to get enough data for the actual statistical work. As a proof-of-concept, however, the study works quite well and should help doctors to better predict whether an expectant mother has to expect birth defects or whether the fetus could be at risk. (Olivier Ami, one of the authors of the study, is also working to develop a software system that can diagnose women at risk of birth defects using MRI scans.)

The researchers also say that their work helps to question the definition of normal birth. Usually defined as a vaginal delivery with minimal effort, they find that such a birth in their study resulted in a baby who was not quite healthy, possibly because his skull had become more deformed. The definition of a normal birth should be updated to include a measure of a child's brain health.