How infants' heads change shape during birth

Using magnetic resonance imaging (MRI), scientists from the University of Clermont Auvergne have captured 3D images that show how infants’ brains and skulls change shape as they move through the birth canal just before delivery. Their work has been published in the journal PLOS ONE.

Doctors have long known that infants’ heads change shape during birth. Termed ‘foetal head moulding’, these changes occur during the second stage of labour, when the baby leaves the uterus and moves through the birth canal. However, the details of foetal head moulding remain unclear, and only one prior study has captured images of this process.

In the latest study, Oliver Ami and colleagues used 3D MRI to capture detailed images of seven infants’ skulls and brains before and during the second stage of labour. The analysis revealed foetal head moulding during the second stage of labour in all seven infants, with different parts of the skull overlapping to varying degrees among the infants.

After birth, five of the newborns’ skull and brain shapes returned to their pre-birth state, but the changes persisted in two of the infants. Two of the three infants with the greatest degree of foetal head moulding were delivered by emergency C-section, but the third was delivered vaginally with minimal expulsive efforts.

Overall, the findings suggest that infants experience greater skull stresses during birth than previously thought, potentially underlying the asymptomatic brain and retinal bleeding seen in many newborns after vaginal delivery. The authors note that a larger study is needed to confirm their findings, but that their work demonstrates the value of 3D MRI in capturing foetal head moulding.

“During vaginal delivery, the foetal brain shape undergoes deformation to varying degrees depending on the degree of overlap of the skull bones,” Ami said. “Foetal skull moulding is no more visible in most newborns after birth. Some skulls accept the deformation (compliance) and allow an easy delivery, while others do not deform easily (non-compliance).”

Image caption: Three-dimensional foetal brain MRI reconstruction before labour (shown in purple in A, C, E) and during the second stage of labour (shown in orange in B, D, F). In all patients, the foetal brain was observed to change in shape in the second stage of labour. This change in foetal brain shape mirrored the shifting of foetal skull bones observed during the second phase of labour. Image courtesy of Ami et al under CC BY 4.0

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