During childbirth, babies leave the warm protection of the womb for the cold, harsh outside world. To survive this transition, we have evolved some astonishing adaptations.
For nine months, the human fetus floats in the womb, quietly growing. Closed off from the outside world, it receives all its nutrients and oxygen through the umbilical cord, and the amniotic fluid in which it bathes. Its tiny lungs lie undeveloped, fluid-filled and deflated. With each passing day, the walls of the womb creep closer and closer, coaxing the fetus towards that decisive moment when it must enter the real world.
Prior to birth, the fetus is surrounded by two transparent, fluid-filled sacs. The inner membrane, the amnion, encases the fetus and contains electrolytes, proteins, carbohydrates and fats donated from the mother’s blood. The outer membrane, the chorion, is part of the placenta and surrounds the amnion.
As the fetus grows, it begins to stretch and strain these membranes. Eventually, the amniotic sac ruptures and spills amniotic fluid out into the womb – the familiar “water breaking”. Childbirth is imminent. The fetus can no longer rely on the amniotic fluid for nutrient exchange; to survive in a gaseous environment, its organs undergo an astonishing physiological transformation.
A newborn, still bathed in amniotic fluid, takes its first breaths. Ernest F/Wikimedia Commons (CC BY-SA 3.0)
As the newborn exits through the birth canal, its fluid-filled lungs are squeezed empty. At the moment of birth, the sudden drop in ambient temperature triggers a neuronal reflex response that causes the baby to sharply inhale. This reflex is maintained throughout life – think about how you respond to jumping into ice-cold water. The baby's ribs snap outwards, causing the lungs to inflate with air.
This first breath is only the beginning. The newborn’s newly-inflated lungs also trigger a dramatic change in its heart anatomy.
The adult human heart comprises four distinct chambers: the left and right atria, and the left and right ventricles. Ordinarily, the right ventricle pumps deoxygenated blood to the lungs, where it absorbs oxygen and returns to the left ventricle to be pumped around the body.
The fetal heart has quite a different configuration. In fetuses, the left and right atria are actually connected via an opening called the foramen ovale. This connection limits blood flow to the lungs to as little as 4% of the total flow. However, as the newborn’s lungs inflate immediately after birth, there is a concurrent drop in lung pressure. This diverts blood to the lungs, snapping the foramen ovale shut and separating the right and left atria of the heart. This ensures that all blood pumped from the right atria is directed towards the newly functioning lungs.
The adult human heart. In the fetal heart, the left and right atria are connected via the foramen ovale. Wapcaplet/Wikimedia Commons (CC BY-SA 3.0)
This entire process occurs within 30 seconds of birth. Over subsequent hours and days, other bodily functions begin to take place.
Exposed to a new harsh and cold environment, the body urgently needs to maintain a warm body temperature. Shortly after birth, the baby burns a specialised type of "brown" fat to produce warmth and maintain a stable body temperature. This, however, is rapidly depleted. Without an umbilical cord and placenta to provide nutrition, a number of changes prepare the baby to absorb new sources of nutrition.
The baby's gastrointestinal tract kicks into gear. Before birth, the gut is filled with a mixture of amniotic fluid, mucus, hair, bile and dead skin and blood cells. Once intestinal contractions begin, this mixture is pushed together into the baby's first stool, known as the meconium.
The adult gastrointestinal tract is filled with a host of bacterial colonies, known as gut microbiota, which help digest food and defend against harmful microorganisms. However, the newborn gastrointestinal tract is sterile, a blank canvas. The microbiota is established within the first hours of birth as the baby is exposed to bacteria from its mother's breast milk and its external environment.
Enterococcus faecalis, a bacterium found in the gastrointestinal tracts of humans and other mammals. United States Department of Agriculture/Wikipedia (public domain)
While some of these foreign bacterial species are beneficial, the majority are not. A newborn baby is extremely susceptible to infection, as its immune system was not needed in the sterile womb. The baby’s immune system must urgently adapt and begin fighting bacterial infections by producing antibodies to combat foreign invaders. Although boosted with antibodies from its mother’s breast milk, the baby’s immune system is one of the most rapidly adapting systems in the body, producing millions of unique antibodies to begin responding to extraneous infections.
These dramatic transformations are just some of the myriad changes that occur within days of birth. Each adaptation is crucial to the health and development of the newborn. Years later, those once-tiny, undeveloped lungs will have grown enough surface area to cover half a tennis court. The heart will be the size of a clenched fist and pump seven litres of blood to the lungs each day, with remarkable consistency and dependability. The once sterile gastrointestinal tract is now home to trillions of microscopic bacteria, helping to extract nutrients our own systems are unable to access.
The precise timing and intricate linkage of the newborn respiratory and circulatory systems is a true marvel of evolution. It exists as a reminder of the miracle of life – a miracle that each and every healthy adult on earth accomplished at birth.
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