For nine months, the human ꜰᴇᴛᴜs floats in the ᴡᴏᴍʙ, quietly growing. Closed off from the outside world, the ꜰᴇᴛᴜs it receives all its nutrients and oxygen through the ᴜᴍʙɪʟɪᴄᴀʟ ᴄᴏʀᴅ, ᴀɴᴅ ᴛʜᴇ ᴀᴍɴɪᴏᴛɪᴄ ꜰʟᴜɪᴅ in which it bathes. Its ᴛɪɴʏ ʟᴜɴɢs lie undeveloped, fluid-filled and deflated. With each passing day, the walls of the ᴡᴏᴍʙ ᴄʀᴇᴇᴘ closer and closer, coaxing the ꜰᴇᴛᴜs towards that decisive moment when it must enter the real world.
Prior to ʙɪʀᴛʜ, the ꜰᴇᴛᴜs is surrounded by two transparent, fluid-filled sacs. The ɪɴɴᴇʀ ᴍᴇᴍʙʀᴀɴᴇ, ᴛʜᴇ ᴀᴍɴɪᴏɴ, ᴇɴᴄᴀsᴇs ᴛʜᴇ ꜰᴇᴛᴜs and contains electrolytes, proteins, carbohydrates and fats donated from the mother’s ʙʟᴏᴏᴅ. The outer membrane, the chorion, is part of the ᴘʟᴀᴄᴇɴᴛᴀ ᴀɴᴅ sᴜʀʀᴏᴜɴᴅs ᴛʜᴇ ᴀᴍɴɪᴏɴ.
As the ꜰᴇᴛᴜs grows, it begins to stretch and strain these membranes. Eventually, the amniotic sac ruptures and spills amniotic fluid out into the ᴡᴏᴍʙ – the familiar “water breaking”. Cʜɪʟᴅʙɪʀᴛʜ is imminent. The ꜰᴇᴛᴜs can no longer rely on the amniotic fluid for nutrient exchange; to sᴜʀᴠɪᴠᴇ in a gaseous environment, its organs undergo an astonishing physiological transformation. As the newborn exits through the ʙɪʀᴛʜ ᴄᴀɴᴀʟ, ɪᴛs ꜰʟᴜɪᴅ-ꜰɪʟʟᴇᴅ ʟᴜɴɢs ᴀʀᴇ sǫᴜᴇᴇᴢᴇᴅ ᴇᴍᴘᴛʏ. At the moment of ʙɪʀᴛʜ, the sudden drop in ambient temperature triggers a ɴᴇᴜʀᴏɴᴀʟ ʀᴇꜰʟᴇx ʀᴇsᴘᴏɴsᴇ ᴛʜᴀᴛ ᴄᴀᴜsᴇs ᴛʜᴇ ʙᴀʙʏ ᴛᴏ sʜᴀʀᴘʟʏ ɪɴʜᴀʟᴇ. This reflex is maintained throughout life – think about how you respond to jumping into ice-cold water. The baby’s ʀɪʙs sɴᴀᴘ ᴏᴜᴛᴡᴀʀᴅs, causing the ʟᴜɴɢs ᴛᴏ ɪɴꜰʟᴀᴛᴇ ᴡɪᴛʜ ᴀɪʀ.
This first breath is only the beginning. The newborn’s ɴᴇᴡʟʏ-ɪɴꜰʟᴀᴛᴇᴅ ʟᴜɴɢs also trigger a dramatic change in its ʜᴇᴀʀᴛ ᴀɴᴀᴛᴏᴍʏ. The adult human ʜᴇᴀʀᴛ comprises four distinct chambers: the left and right atria, and the left and right ventricles. Ordinarily, the right ventricle pumps deoxygenated ʙʟᴏᴏᴅ to the ʟᴜɴɢs, where it absorbs oxygen and returns to the left ventricle to be pumped around the body.
The ꜰᴇᴛᴀʟ ʜᴇᴀʀᴛ has quite a different configuration. In ꜰᴇᴛᴜses, the left and right atria are actually connected via an opening called the foramen ovale. This connection limits ʙʟᴏᴏᴅ flow to the ʟᴜɴɢs to as little as 4% of the total flow. However, as the newborn’s ʟᴜɴɢs inflate immediately after ʙɪʀᴛʜ, there is a concurrent drop in lung pressure. This diverts ʙʟᴏᴏᴅ to the ʟᴜɴɢs, snapping the foramen ovale shut and separating the right and left atria of the ʜᴇᴀʀᴛ. This ensures that all ʙʟᴏᴏᴅ pumped from the right atria is directed towards the newly functioning ʟᴜɴɢs. This entire process occurs within 30 seconds of ʙɪʀᴛʜ. Over subsequent hours and days, other bodily functions begin to take place.
Exposed to a new ʜᴀʀsʜ and cold environment, the body urgently needs to maintain a warm body temperature. Shortly after ʙɪʀᴛʜ, the baby burns a specialised type of “brown” fat to produce warmth and maintain a stable body temperature. This, however, is rapidly ᴅᴇᴘʟᴇᴛᴇᴅ. Without an ᴜᴍʙɪʟɪᴄᴀʟ ᴄᴏʀᴅ ᴀɴᴅ ᴘʟᴀᴄᴇɴᴛᴀ 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 ʙɪʀᴛʜ, the ɢᴜᴛ ɪs ꜰɪʟʟᴇᴅ ᴡɪᴛʜ ᴀ ᴍɪxᴛᴜʀᴇ ᴏꜰ ᴀᴍɴɪᴏᴛɪᴄ ꜰʟᴜɪᴅ, ᴍᴜᴄᴜs, ʜᴀɪʀ, ʙɪʟᴇ ᴀɴᴅ ᴅᴇᴀᴅ sᴋɪɴ ᴀɴᴅ ʙʟᴏᴏᴅ ᴄᴇʟʟs. Once ɪɴᴛᴇsᴛɪɴᴀʟ ᴄᴏɴᴛʀᴀᴄᴛɪᴏɴs ʙᴇɢɪɴ, this mixture is pushed together into the baby’s first stool, known as the meconium.
The newborn gastrointestinal tract is sterile, a blank canvas. The microbiota is established within the first hours of ʙɪʀᴛʜ as the baby is exposed to bacteria from its mother’s breast milk and its external environment. While some of these foreign bacterial species are beneficial, the majority are not. A newborn baby is ᴇxᴛʀᴇᴍᴇʟʏ sᴜsᴄᴇᴘᴛɪʙʟᴇ ᴛᴏ ɪɴꜰᴇᴄᴛɪᴏɴ, as its immune system was not needed in the sterile ᴡᴏᴍʙ. The baby’s immune system must urgently adapt and begin ꜰɪɢʜᴛɪɴɢ ʙᴀᴄᴛᴇʀɪᴀʟ ɪɴꜰᴇᴄᴛɪᴏɴs by producing antibodies to combat foreign ɪɴᴠᴀᴅᴇʀs. 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 ᴇxᴛʀᴀɴᴇᴏᴜs ɪɴꜰᴇᴄᴛɪᴏɴs.
These dramatic transformations are just some of the myriad changes that occur within days of ʙɪʀᴛʜ. Each adaptation is crucial to the health and development of the newborn. Years later, those once-tiny, ᴜɴᴅᴇᴠᴇʟᴏᴘᴇᴅ ʟᴜɴɢs will have grown enough surface area to cover half a tennis court. The ʜᴇᴀʀᴛ will be the size of a clenched fist and pump seven litres of ʙʟᴏᴏᴅ to the ʟᴜɴɢs 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 ʀᴇsᴘɪʀᴀᴛᴏʀʏ ᴀɴᴅ ᴄɪʀᴄᴜʟᴀᴛᴏʀʏ 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 ʙɪʀᴛʜ.