The Science of Birth: Understanding the Physiology Behind Labour and Birth

The journey of birth is one of the most profound events in human life, deeply rooted in a

complex series of physiological processes. These processes have evolved to ensure the safety

of both the mother and the baby. Understanding the science behind labour can demystify this

incredible event and empower families as they prepare to welcome a new life.

The History of Birth and Why Humans Birth Differently

Humans have one of the most unique and challenging birthing processes compared to other

mammals. Our evolution has influenced not only the physiology of labour but also the social and

cultural significance of birth. Understanding the history of birth can provide insight into why

human birth is often more complex than that of other animals.

The Evolution of Human Birth

The primary reason humans birth differently than other mammals is our evolutionary trade-off

between bipedalism and brain size. About 4 to 6 million years ago, early humans evolved to

walk upright on two legs. While this adaptation offered significant advantages in mobility, it also

led to a narrowing of the pelvis, particularly the birth canal.

At the same time, human brains grew dramatically in size, particularly during the later stages of

evolution. A large brain offers incredible cognitive abilities, but it also means a much larger head

that must pass through the birth canal. These two evolutionary pressures—bipedalism and large

brain size—created what is sometimes called the “obstetric dilemma.” This dilemma makes

human labour and birth more challenging compared to other mammals, whose infants typically

have smaller brains relative to their body size and are born more quickly and easily.

The Importance of Social Birth Support

To overcome the challenges of birthing large-headed babies through a narrower pelvis, humans

have developed unique social behaviours. Unlike many mammals that birth alone, human birth

has often been a social event. Anthropological evidence shows that even early humans relied

on others—such as midwives, family members, or community members—to assist during birth.

This communal support helped manage the risks and difficulties of childbirth.

Historically, birth was a family or community-centred experience. Midwifery has ancient roots,

with evidence of midwives assisting births dating back thousands of years across different

cultures. In ancient Egypt, midwives were highly respected, and in medieval Europe, the

presence of midwives was essential for childbirth. Throughout much of human history, birth took

place at home, surrounded by loved ones and birth attendants. It was only in the last couple of

centuries, with the rise of modern medicine, that hospital births became more common,

particularly in Western societies.

The Stages of Labour

Labour is typically divided into three stages:

1. The First Stage: Early Labour, Active Labour, and Transition

2. The Second Stage: The Birth of the Baby

3. The Third Stage: Delivery of the Placenta

Each stage is driven by a series of hormonal changes and physical processes.

Hormonal Orchestration of Labour

Hormones play a crucial role in every phase of labour. The most prominent are oxytocin,

prostaglandins, and endorphins, which work in concert to regulate contractions, promote

cervical dilation, and manage pain.

● Oxytocin is often referred to as the “love hormone,” but during labour, it serves a critical

function in stimulating uterine contractions. These contractions gradually increase in

intensity and frequency, helping the cervix thin (efface) and open (dilate) to prepare for

birth.

● Prostaglandins help soften and ripen the cervix in preparation for delivery. These

hormone-like compounds are released in response to signals from the baby and the

body’s readiness for birth.

● Endorphins are the body’s natural pain-relieving hormones. As labour progresses and

becomes more intense, endorphin levels rise, helping the birthing person manage

discomfort and stress.

The Uterus and Cervix: Powerhouses of Labour

The uterus is a powerful muscle designed to contract and expel the baby during labour.

Throughout pregnancy, the cervix remains closed and firm, acting as a barrier between the baby

and the outside world. As labour begins, hormonal changes trigger the cervix to soften, thin, and

dilate.

● Effacement refers to the thinning of the cervix. The cervix is typically about 3-4 cm thick,

but during labour, it becomes paper-thin to allow the baby to pass through the birth

canal.

● Dilation is the opening of the cervix, measured in centimetres from 0 to 10. Full dilation

(10 cm) allows the baby’s head to pass through the cervix and into the birth canal.

The Phases of the First Stage

The first stage of labour is divided into three phases: early labour, active labour, and transition.

● Early labour involves mild, irregular contractions that help with the early effacement and

dilation of the cervix, usually up to 6 cm. This phase can last for several hours or even

days.

● Active labour is marked by stronger, more regular contractions that progressively dilate

the cervix from 6 cm to 8 cm. During this phase, the birthing person will likely feel more

intense discomfort as the body prepares for birth.

● Transition is the final phase of the first stage and often considered the most intense. It

occurs when the cervix dilates from 8 cm to 10 cm. Contractions are typically very strong

and frequent, coming every two to three minutes and lasting for up to 90 seconds.

Emotionally and physically, this phase can feel overwhelming, but it is also the shortest

phase, often lasting from 15 minutes to an hour. Once the cervix reaches 10 cm, the

mother is ready to begin pushing.

The Second Stage: The Birth of the Baby

Once the cervix is fully dilated, the second stage of labour begins. At this point, the birthing

person will experience powerful contractions that help push the baby down the birth canal.

● Fetal Descent: The baby’s head engages in the pelvis and moves down through the

birth canal. As the head crowns (emerges at the vaginal opening), the birthing person

may feel a stretching or burning sensation, often called the “ring of fire.”

● Bearing Down: This stage is often characterized by an urge to push. The pushing

efforts, combined with uterine contractions, help guide the baby through the birth canal

and out into the world.

The Third Stage: Delivery of the Placenta

After the baby is born, the body continues its work by delivering the placenta. This stage is

usually brief, lasting anywhere from 5 to 30 minutes. The uterus continues to contract, helping to

detach the placenta from the uterine wall. Once the placenta is delivered, these contractions

help reduce postpartum bleeding by compressing the blood vessels in the uterus.

The Baby’s Role in Labour

Interestingly, the baby also plays an active role in initiating labour. Research shows that as the

baby’s lungs mature, they release proteins into the amniotic fluid that signal the body to begin

labour. The baby’s position in the uterus is also crucial. Ideally, the baby should be in a

head-down, anterior position (facing the mother’s back) for a smoother birth. However, even in

less optimal positions, such as breech or posterior, the body adapts to facilitate the birth

process.

The Importance of the Mind-Body Connection

Labour is not only a physical process but also an emotional and psychological one. The

mind-body connection plays a significant role in how labour progresses. Stress and anxiety can

interfere with the natural flow of hormones like oxytocin, potentially slowing labour. This is why

creating a calm, supportive environment during labour is crucial. Relaxation techniques such as

deep breathing, visualization, and continuous support from a doula or loved ones can help keep

oxytocin levels high, promoting a smoother labour.

The Gate Control Theory of Pain

The experience of pain during labour is highly individual, but science has offered insight into

how the body processes pain signals. According to the Gate Control Theory of Pain, the spinal

cord contains a neurological “gate” that either allows or blocks pain signals from reaching the

brain. Non-painful input, such as massage or warmth, can “close the gate,” reducing the

perception of pain. This is why many birthing people find relief through comfort measures like

water immersion, massage, or counterpressure during contractions.

The Incredible Adaptability of the Human Body

The human body’s ability to adapt to labour and birth is nothing short of miraculous. From the

hormonal cascade that initiates labour to the physical transformation of the uterus and cervix,

every part of the birthing process is designed to bring new life into the world safely.

Understanding these physiological mechanisms can help families approach birth with

confidence, knowing that their bodies are built for this transformative experience.

While birth is a natural process, it is important to remember that every birth is unique, and

sometimes medical intervention is necessary. Whether labour progresses smoothly or requires

assistance, the physiology of birth remains a powerful testament to the resilience and strength

of the human body.

By understanding the science of birth, we can better appreciate the wonder of this experience

and support families as they navigate this incredible journey. Whether through natural labour or

medical support, the physiological processes that guide birth are a reminder of the intricate

design that makes human life possible.

Vanessa - Birth Doula