3D images reveal how much infants’ heads deform during birth

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When babies pass through the mother's birth canal, the tight fit temporarily squashes their heads, elongating their flexible skulls and changing the shape of their brains. Now, scientists have created 3D images that demonstrate the extent of that amazing conehead-like distortion.

Babies' heads can change shape under pressure because the bones in their skulls haven't fused together yet. Soft regions at the top of the head accommodate being squeezed through the birth canal and allow room for the brain to grow during infancy.

However, the precise mechanics of how a baby's skull and brain change shape during labour are not well understood.

For a new study, “Three-dimensional magnetic resonance imaging of foetal head moulding and brain shape changes during the second stage of labour”, published in PLOS One, medical experts used 3D MRI to capture remarkably detailed images of babies’ skulls and brains during advanced stages of labour. Their findings suggest that infants’ heads undergo considerable stress during birth, more so than experts had previously thought.

To learn more about that process, 27 pregnant women consented to receiving MRI scans during weeks 36 and 39 of their pregnancies, and of those, seven agreed to be scanned during the second stage of labour - during which their cervix has dilated to 10cm and the baby is born. The imaging was performed no more than ten minutes before "expulsory effort," or when the baby descends into the birth canal and mother can begin to push. After the images were taken, the mothers were swiftly rushed to the delivery room; “Patient transportation time from the MRI suite to the delivery room in the same building, bed to bed, was less than three minutes,” the study authors note.

Upon comparing the pre-labour and mid-labour images they revealed significant skull squeezing, known as foetal head moulding, in all of the infants, and suggested that the pressures exerted on infant heads and brains during birth are stronger than once thought, scientists reported in a new study. This means that different parts of the skull overlapped, to varying degrees, during the birthing process. Infants’ skulls are thus comprised of several bony sections, held together by fibrous materials called sutures, that eventually fuse as the baby grows outside the womb.

Three-dimensional finite element reconstruction of the cranial bones before labour and during the second stage of labour. Image credit: Ami et al., 2019

Researchers know that skull shifting during birth has been happening in humans and their ancestors for millions of years. It's an adaptation to the evolution of larger brains and the switch to upright walking, which altered the shape of the pelvis. Still, the researchers were surprised by just how much babies’ heads were deforming as they moved through the birth canal.

“When we showed the foetal head changing shape, we discovered that we had underestimated a lot of the brain compression during birth,” said study author Olivier Ami, an obstetrician and gynaecologist at University of Clermont Auvergne in France.

The skulls of five of the babies under observation quickly returned to their pre-birth state, but changes persisted in two of the babies, possibly due to differences in the elasticity of the skull bones and the supporting fibrous material, among other factors. Two of the three babies with the largest degree of head moulding still needed to be delivered via caesarean section, indicating that mothers may not always be able to give birth vaginally, “even when significant foetal moulding occurs,” the study authors note.

Interestingly, the third baby among those with the highest degrees of head warping initially scored low on the Apgar test, which is given to babies soon after birth and assesses skin colour, pulse, reflexes, muscle tone and breathing rate. By the time the baby was 10 minutes old, however, its score had risen to a perfect 10. The researchers do not yet know how or if ease of delivery — the infant was born vaginally and the delivery was “uncomplicated” — and foetal head moulding factors into this “risky clinical presentation,” the study authors note. 

But it does suggest a rethink is warranted on what are considered “normal births”, which are typically defined as natural births that happen with “only a few maternal expulsive efforts.”

“This definition does not take into consideration the ability of the foetal head to deform. If the foetal head’s compliance (deformation) is high, the skull and brain may undergo significant deformation as the birth canal is crossed, and the child's condition at birth may not be good,” the researchers explain.

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

Revelations about the stresses that come with foetal head moulding might also explain why some babies are born with retinal and brain haemorrhages, the latter of which can lead to complications like cerebral palsy, Edwards reports. And though the study is small, the researchers say the high quality imaging could inform efforts to develop a “more realistic simulation of delivery” that will help medical experts predict which mothers are at risk of running into biomechanical complications during childbirth, and intervene before harm comes to the baby.

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.