Giving birth humans

Scientists Have Figured Out Why Childbirth Became So Complex and Dangerous

The World Health Organization estimates that nearly 300,000 people die every year due to pregnancy-related causes.

A study finds that complex human childbirth and cognitive abilities are a result of walking upright.

Childbirth in humans is much more complex and painful than in great apes. It was long believed that this was a result of humans’ larger brains and the narrow dimensions of the mother’s pelvis. Researchers at the University of Zurich have now used 3D simulations to show that childbirth was also a highly complex process in early hominin species that gave birth to relatively small-brained newborns – with important implications for their cognitive development.

Complications are common for women during and following pregnancy and childbirth. The majority of these issues arise during pregnancy and are either avoidable or curable. However, childbirth is still dangerous. The World Health Organization estimates that 830 people die every day due to causes related to childbirth and pregnancy. Furthermore, for every woman that dies due to childbirth, another 20-30 encounter injury, infection, or disabilities.

Four major complications are responsible for 75% of maternal deaths: severe bleeding (typically after birth), infections, high blood pressure during pregnancy, and complications from delivery. Other common issues include unsafe abortions and chronic conditions such as cardiac diseases and diabetes.

All of this shows how human birthing is much more difficult and painful than that of large apes. This was long believed to be due to humans’ bigger brains and the limited dimensions of the mother’s pelvis. Researchers at the University of Zurich have now shown, using 3D simulations, that birthing was likewise a highly complicated procedure in early hominin species that gave birth to relatively small-brained newborns – with significant consequences for their cognitive development.

The fetus normally navigates a narrow, convoluted birth canal by bending and turning its head at different phases during human delivery. This complicated procedure has a significant risk of birth complications, which may range from extended labor to stillbirth or maternal death. These issues were long thought to be the outcome of a conflict between humans adjusting to upright walking and our larger brains.

The dilemma between walking upright and larger brains

Bipedalism developed around seven million years ago and dramatically reshaped the hominin pelvis into a real birth canal. Larger brains, however, didn’t start to develop until two million years ago, when the earliest species of the genus Homo emerged. The evolutionary solution to the dilemma brought about by these two conflicting evolutionary forces was to give birth to neurologically immature and helpless newborns with relatively small brains – a condition known as secondary altriciality.

A research group led by Martin Häusler from the Institute of Evolutionary Medicine at the University of Zurich (UZH) and a team headed up by Pierre Frémondière from Aix-Marseille University have now found that australopithecines, who lived about four to two million years ago, had a complex birth pattern compared to great apes. “Because australopithecines such as Lucy had relatively small brain sizes but already displayed morphological adaptations to bipedalism, they are ideal to investigate the effects of these two conflicting evolutionary forces,” Häusler says.

Birth simulation of Lucy (Australopithecus afarensis) with three different fetal head sizes. Only a brain size of maximum 30 percent of the adult size (right) fits through the birth canal. Credit: Martin Häusler, UZH

The typical ratio of fetal and adult head size

The researchers used three-dimensional computer simulations to develop their findings. Since no fossils of newborn australopithecines are known to exist, they simulated the birth process using different fetal head sizes to take into account the possible range of estimates. Every species has a typical ratio between the brain sizes of its newborns and adults. Based on the ratio of non-human primates and the average brain size of an adult Australopithecus, the researchers calculated a mean neonatal brain size of 180 g. This would correspond to a size of 110 g in humans.

For their 3D simulations, the researchers also took into account the increased pelvic joint mobility during pregnancy and determined a realistic soft tissue thickness. They found that only the 110 g fetal head sizes passed through the pelvic inlet and midplane without difficulty, unlike the 180 g and 145 g sizes. “This means that Australopithecus newborns were neurologically immature and dependent on help, similar to human babies today,” Häusler explains.

Prolonged learning is key to cognitive and cultural abilities

The findings indicate that australopithecines are likely to have practiced a form of cooperative breeding, even before the genus Homo appeared. Compared to great apes, the brains developed for longer outside the uterus, enabling infants to learn from other members of the group. “This prolonged period of learning is generally considered crucial for the cognitive and cultural development of humans,” Häusler says. This conclusion is also supported by the earliest documented stone tools, which date back to 3.3 million years ago – long before the genus Homo appeared.

Reference: “Dynamic finite-element simulations reveal early origin of complex human birth pattern” by Pierre Frémondière, Lionel Thollon, François Marchal, Cinzia Fornai, Nicole M. Webb, and Martin Haeusler, 19 April 2022, Communications Biology.
DOI: 10.1038/s42003-022-03321-z

What happens to your body during childbirth

Childbirth is challenging and complications occur, but women's bodies are designed to give birth. The shape of the pelvis, hormones, powerful muscles and more all work together to help you bring your baby into the world - before, during and after childbirth.

How your body prepares for labour

Here are some of the ways your body will prepare both you and your baby for the birth ahead.

Braxton Hicks contractions

In the weeks or days before you start having proper contractions, you may experience Braxton Hicks contractions. This is your uterus tightening then relaxing. These contractions don't usually hurt and are thought to help your uterus and cervix get ready for labour.

Braxton Hicks contractions may become more regular as you get closer to the time of birth, but unlike labour contractions, they don't change the shape of the cervix and are sometimes referred to as 'false labour'. Your midwife can tell you if you're experiencing Braxton Hicks contractions or if you are in labour by doing a vaginal examination to look at your cervix.

Changes to the cervix

As labour gets closer, your cervix softens and becomes thinner, getting ready for the dilation (widening) that will allow the baby to enter the vagina. You may also see a 'show', which is a pinkish plug of mucus, stained with blood.

Engagement

Your baby may move further down your pelvis as the head engages, or sits in place over your cervix, ready for the birth. Some women feel they have more room to breathe after the baby has moved down. This is called 'lightening'.

Rupture of the membranes, or 'waters breaking'

Some women find the sac of amniotic fluid containing the baby breaks before labour, contractions start and the fluid runs (or gushes) out of the vagina. This is referred to as rupture of the membranes, or 'waters breaking'.

Let your maternity team know when your waters have broken and take notice of the colour of the fluid. It is usually light yellow. If it is green or red, tell your maternity team since this could mean the baby is having problems.

If your waters have broken but you have not started having regular contractions within 24 hours, you may need your labour to be induced because there is a risk of infection. Your midwife or doctor will talk to you about this.

How will you know when labour has started?

Movies often show women suddenly being struck by painful contractions and rushing to hospital. In real life, many women are not sure if they have actually started their labour.

You may feel restless, have back pain or period-like pain, or stomach disturbances such as diarrhoea.

Labour officially begins with contractions, which start working to open up the cervix. You should phone your midwife when your contractions start, although you probably won't be encouraged to come to the hospital or birthing centre until your contractions are closer together.

In preparation for labour, your baby may move further down your pelvis as the head engages, or sits in place over your cervix.

How the pelvis is designed for childbirth

Your pelvis is located between your hip bones. Women typically have wider, flatter pelvises than men, as well as a wider pelvic cavity (hole) to allow a baby to pass through.

The organs sitting in a woman's pelvis include the uterus, cervix and vagina, which are held together by a group of muscles. During childbirth, the muscles at the top of your uterus press down on the baby's bottom. Your baby's head then presses on your cervix which, along with the release of the hormone oxytocin (see 'How hormones help you give birth', below), brings on contractions. Your cervix should dilate so your baby can pass through it.

Your pelvis has bones and ligaments that move or stretch as the baby travels into the vagina. Your baby also has spaces between the skull bones called 'sutures', and the gaps where the sutures meet on the skull are called fontanelles. This allows for the baby's head to mould as the skull bones meet or overlap, allowing it to fit more easily as it travels through your pelvis.

How hormones help you give birth

Your body produces hormones that trigger changes in your body before, during and after childbirth. Here's how they work to help you deliver your baby.

Prostaglandin Before childbirth, a higher level of prostaglandin will help open the cervix and make your body more receptive to another important hormone, oxytocin.

Oxytocin This hormone causes contractions during labour, as well as the contractions that deliver the placenta after the baby is born. These post-birth contractions, including more that can occur during breastfeeding, help your uterus shrink back to its normal size. Oxytocin and prolactin are the two main hormones that produce and let down breast milk for your baby. Skin-to-skin contact between a mother and baby helps to release more of these hormones.

Relaxin The hormone relaxin helps soften and stretch the cervix for birth, while helping your waters break and stretching the ligaments in your pelvis to allow the baby to come through.

Beta-endorphins During childbirth, this type of endorphin helps with pain relief and can cause you to feel joyful or euphoric.

'Baby blues' After birth, your hormone balance can change again, and this is believed to cause the ‘baby blues’ in some women. You may feel teary, anxious and irritable and your mood can go up and down.

When childbirth doesn’t go to plan

Sometimes, complications can occur before or during childbirth that mean things don’t go as expected.

Sometimes, labour needs to be induced or started. There are a few ways to induce labour, including the mother being offered synthetic prostaglandin. This is inserted into the vagina to soften the cervix and start contractions.

If contractions slow down or stop during labour, the mother may be offered synthetic oxytocin from a drip to increase the contractions. In both these cases contractions can come on strongly and more pain relief may be needed. Your maternity team should explain the benefits and risks of this with you before you agree to it.

The baby could be in a posterior or breech position, not ideally placed above the cervix before the birth. Your maternity team may need to use forceps or a vacuum to help turn the baby or help the baby travel out of the vagina. Sometimes a caesarean is needed.

In rare cases, a mother may experience cephalopelvic disproportion (CPD), which is when the baby’s head is too big to fit through the pelvis. A diagnosis of CPD is usually made when labour hasn’t progressed and synthetic oxytocin has not helped. A caesarean is usually the next step.

More information

If you have any questions about childbirth or pregnancy, you can call Pregnancy, Birth and Baby on 1800 882 436, 7 days a week, to speak to a maternal health nurse.

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