Babies in the womb never take a single breath of air. Every molecule of oxygen they need arrives through the placenta, which functions as a stand-in for the lungs until birth. The mother’s blood delivers oxygen to the placental tissue, and from there it crosses into the baby’s bloodstream and travels through an elaborate circulatory system that looks nothing like the one we use after birth. The whole arrangement is so effective that a fetus can grow from a cluster of cells to a full-sized newborn without its lungs ever doing the job they were built for.
The Placenta Works Like a Lung
The placenta is the organ that makes fetal life possible. Anchored to the uterine wall and connected to the baby through the umbilical cord, it handles gas exchange, nutrient delivery, and waste removal all at once. Maternal blood flows into spaces within the placenta, and fetal blood flows through tiny vessels called villi that project into those spaces. Oxygen moves from the mother’s blood across the thin villous membrane and into the fetal blood, while carbon dioxide travels in the opposite direction. The two bloodstreams never actually mix; the transfer happens by diffusion across the tissue barrier, much the way oxygen crosses the walls of air sacs in your lungs.
The placenta is less efficient at gas exchange than a pair of healthy adult lungs, which means the fetus operates at oxygen levels that would make an adult feel like they were on a high mountain. A fetus thrives at these levels partly because its metabolism is tuned for it and partly because of a molecular trick in its blood.
Fetal Hemoglobin Grabs Oxygen More Tightly
The hemoglobin in fetal red blood cells is structurally different from the adult version. Fetal hemoglobin has a higher affinity for oxygen, meaning it binds oxygen more readily and holds on more tightly at the low oxygen concentrations found in placental blood. This is critical: fetal hemoglobin essentially pulls oxygen away from the mother’s hemoglobin across the placental barrier. Research on the biophysical properties of fetal hemoglobin shows that its internal structural balance is shifted in a way that favors oxygen binding compared to adult hemoglobin under the same conditions.1PubMed. The effect of organic cosolvents on the oxygen affinity of fetal hemoglobin. Relevance of protein-solvent interactions to the functional properties After birth, fetal hemoglobin is gradually replaced by the adult form over the first several months of life, as the baby’s lungs take over and higher oxygen levels become the norm.
A Circulation Built for Life Without Lungs
The fetal circulatory system has special shortcuts that reroute blood away from the lungs, which are filled with fluid and not yet in use. Two of the most important are the foramen ovale (a small opening between the left and right upper chambers of the heart) and the ductus arteriosus (a vessel connecting the pulmonary artery to the aorta). Together, these detours allow most of the blood to bypass the lungs entirely and head straight to the body.
The routing is surprisingly precise. An ultrasound study of fetal blood flow identified two distinct pathways: a left-sided pathway that channels oxygenated blood from the umbilical vein through the ductus venosus and directly through the foramen ovale into the left side of the heart, and a right-sided pathway that sends less oxygenated blood into the right atrium. This arrangement means the most oxygen-rich blood is preferentially directed to the brain and heart, while less oxygenated blood is sent to the lower body and back to the placenta for a fresh supply.2PubMed. Foramen ovale: an ultrasonographic study of its relation to the inferior vena cava, ductus venosus and hepatic veins It is an elegant piece of biological engineering that disappears within hours of birth, when the foramen ovale closes and the ductus arteriosus constricts, sending blood through the now-functional lungs for the first time.
Practice Breathing That Does Not Involve Oxygen
Even though a fetus gets all its oxygen from the placenta, it still makes rhythmic breathing-like movements in the womb. These fetal breathing movements are not about gas exchange at all. Instead, they serve as rehearsal: they help the lungs grow, strengthen the muscles of the diaphragm and chest wall, and train the neural circuits that will control breathing after birth.3PubMed. Fetal breathing movements and changes at birth
What the fetus “inhales” and “exhales” during these movements is amniotic fluid, not air. Studies in sheep show that fetal breathing movements begin remarkably early, around 40 days of gestation in sheep. Early on they are nearly continuous, but later in pregnancy they become episodic, occurring in bursts rather than steadily.4PubMed Central. The central control of fetal breathing and skeletal muscle movements In human pregnancies, expectant parents sometimes notice these movements on ultrasound as a gentle rhythmic flickering of the baby’s chest. Healthcare providers consider the presence of fetal breathing movements a sign of well-being; their absence can sometimes indicate fetal distress.
How the Lungs Prepare for Air
Although the lungs sit idle during pregnancy, they are far from inactive. The fetus is quietly building and maturing them in preparation for birth. Two elements are especially important: amniotic fluid pressure and surfactant.
The lungs are filled with fluid throughout fetal life, and the surrounding amniotic fluid plays a key role in keeping them properly inflated. Adequate amniotic fluid levels help transmit pressure to the developing airways and air sacs, which is critical for normal lung growth.5PubMed Central. Amniotic Fluid: Its Role in Fetal Development and Beyond – Section: Lung development When amniotic fluid is too low, a condition called oligohydramnios, the lungs can end up dangerously underdeveloped. That relationship between fluid levels and lung size has been confirmed in both animal experiments and population studies.6PubMed. Pulmonary Hypoplasia Induced by Oligohydramnios: Findings from Animal Models and a Population-Based Study
The other critical preparation is surfactant, a slippery mixture of fats and proteins produced by specialized cells in the lung lining. Surfactant coats the inside of the air sacs and keeps them from collapsing when you exhale. Without it, every breath would be like trying to peel apart two wet sheets of glass. Surfactant production ramps up as the fetus approaches full term. Hormones called glucocorticoids play a central role in triggering this surge, stimulating the lung cells to produce the key proteins and fats.7PubMed. Glucocorticoids and lung development in the fetus and preterm infant This is why premature babies often struggle to breathe: their lungs have not had time to make enough surfactant. When doctors know a baby will be born early, they frequently give the mother corticosteroid injections to speed up surfactant production before delivery.8Cambridge University Press. Surfactant during lung development – Section: Abstract
The First Breath
Birth triggers one of the most dramatic physiological transitions in all of biology. Within seconds of delivery, the baby must switch from placental oxygen to breathing air. The lungs, which have been filled with fluid for months, need to clear that fluid and inflate with air for the first time. Several forces drive this: the physical compression of the chest during vaginal delivery squeezes out some fluid, and specialized channels in the lung lining actively pump the rest into the bloodstream. The baby’s first cry helps too, generating the pressure needed to pop open millions of tiny air sacs that have never held air before.
At the same time, the circulatory shortcuts that bypassed the lungs begin to close. As the lungs fill with air and blood oxygen levels rise, blood vessels in the lungs relax, blood flow through them surges, and the foramen ovale and ductus arteriosus seal shut over the first hours and days. The system that served the fetus so well in the womb is dismantled and replaced by the adult pattern in a matter of minutes. When this transition goes smoothly, it is something most parents never think about. When it doesn’t, it can become a medical emergency.
When Oxygen Runs Low Before Birth
The fetus is not defenseless if its oxygen supply drops. When oxygen levels fall, whether from a temporary kink in the umbilical cord, a contraction during labor, or a problem with the placenta, the fetus has a built-in emergency response. The cardiovascular system shifts gears: blood flow to the skin, gut, and limbs is reduced, and a larger share is redirected to the brain and heart. This “brain-sparing” response is triggered by oxygen sensors in the carotid arteries and is further modulated by stress hormones and changes in blood vessel tone.9PubMed Central. The fetal brain sparing response to hypoxia: physiological mechanisms During acute oxygen deprivation, the proportion of placental blood routed through the ductus venosus also increases, ensuring that whatever oxygenated blood is available reaches the most critical organs as efficiently as possible.10PubMed. Cardiac output and blood flow volume redistribution during acute maternal hypoxia in fetal sheep
This response is effective for short episodes, but prolonged or severe oxygen deprivation can overwhelm the system. If the fetus becomes severely hypoxic, it may start gasping reflexively, which can draw meconium (the baby’s first stool, sometimes released into the amniotic fluid under stress) into the airways. This is known as meconium aspiration syndrome and can cause serious breathing problems after birth.11Obstetrics, Gynaecology & Reproductive Medicine. Meconium aspiration syndrome – Section: Abstract
How Maternal Health Affects Fetal Oxygen
Because the entire oxygen supply chain runs through the mother, anything that compromises her circulation or blood oxygen has a direct effect on the baby. Smoking during pregnancy is one of the clearest examples. Nicotine causes blood vessels to constrict, reducing blood flow to the placenta, while carbon monoxide from cigarette smoke binds to hemoglobin and lowers the amount of oxygen the blood can carry. The result is chronic, low-grade oxygen deprivation to the fetus, which contributes to placental insufficiency and smaller birth weight.12PubMed Central. Impact of Maternal Smoking on Obstetric and Neonatal Outcomes in Twin Pregnancies: A Narrative Review – Section: Abstract
Living at high altitude presents a different kind of challenge. At elevations above roughly 2,500 meters, the air contains less oxygen, which means less oxygen reaches the mother’s blood and therefore the placenta. Populations that have lived at high altitude for generations show adaptations in placental structure that appear to benefit oxygen diffusion, helping compensate for the thinner air.13PubMed Central. Humans at high altitude: hypoxia and fetal growth Babies born at high altitude tend to be somewhat smaller on average than those born at sea level, though in well-adapted populations the difference is less pronounced. Conditions like preeclampsia and gestational diabetes also impair placental function and can reduce fetal oxygen supply, which is one reason prenatal monitoring focuses so heavily on placental health.
What Happens When Lungs Do Not Develop Properly
The connection between amniotic fluid, fetal breathing movements, and lung growth means that problems with any one of these can cascade. When amniotic fluid is severely reduced, two things go wrong at once: the mechanical pressure that helps keep the developing lungs inflated is lost, and the fetal breathing movements that promote lung growth are impaired. Animal experiments have teased apart these two contributions. Even when fetal breathing was completely eliminated by cutting the nerve to the diaphragm, further draining of amniotic fluid still caused additional lung shrinkage, showing that the internal fluid pressure itself is an independent factor in keeping lungs growing.14PubMed. Experimental pulmonary hypoplasia and oligohydramnios: relative contributions of lung fluid and fetal breathing movements
The resulting condition, pulmonary hypoplasia (underdeveloped lungs), is a serious cause of death in newborns. Research has also found that low amniotic fluid reduces the expression of growth-signaling proteins in fetal lung tissue, suggesting the damage is not purely mechanical but also molecular.15PubMed. Oligohydramnios decreases platelet-derived growth factor expression in fetal rat lungs This is why obstetricians pay close attention to amniotic fluid levels throughout pregnancy. Very low fluid, especially early or in the middle of pregnancy, is a red flag for potential lung problems at birth.
Sharing a Placenta in Twin Pregnancies
Identical twins who share a single placenta, called monochorionic twins, face unique challenges to their oxygen supply. Their blood vessels often connect across the placental surface through natural anastomoses, or junctions. A study of monochorionic twin placentas found vascular connections in nearly nine out of ten cases.16PubMed. Vascular anastomoses in intrauterine growth in monochorionic twins Most of the time these connections actually help, balancing blood flow between the twins and compensating when one twin’s share of the placental territory is smaller. Twins with at least one artery-to-artery connection tended to have similar birth weights even when their placental territories were unequal.
But when the connections are imbalanced, one twin can end up pumping blood to the other with no adequate return flow. This is twin-to-twin transfusion syndrome, which occurs in roughly one in ten monochorionic twin pregnancies. One twin becomes overloaded with blood while the other becomes depleted, leading to serious complications for both, including potential heart failure and growth restriction.17PubMed Central. Twin to twin transfusion syndrome Treatment typically involves laser surgery to seal off the problematic blood vessel connections on the placental surface.
The Artificial Placenta
One of the more ambitious frontiers in neonatal medicine is the development of an artificial placenta, a device that would support extremely premature infants by mimicking fetal physiology rather than forcing immature lungs to breathe air. The concept goes back decades. In 1969, researchers kept a premature lamb fetus alive for several days using a membrane oxygenator and intravenous nutrition, essentially replacing the placenta with a machine.18PubMed. Artificial placenta: two days of total extrauterine support of the isolated premature lamb fetus
Modern efforts have advanced considerably. Two distinct platforms are under active development: one uses the baby’s own heartbeat to drive blood through an external oxygenator (an arteriovenous approach), while the other uses a pump (a veno-venous approach). Both keep the infant submerged in artificial amniotic fluid to protect the developing lungs and skin while the oxygenator handles gas exchange the way the placenta would.19PubMed Central. Development of an artificial placenta for support of premature infants: narrative review of the history, recent milestones, and future innovation – Section: Current AP systems The technology is still in animal testing, but if it succeeds in human trials, it could transform the care of the most vulnerable premature infants by giving their lungs more time to mature before they ever have to take a breath.
How We Figured This Out
Humans have wondered how fetuses survive without breathing for thousands of years. Aristotle speculated about a vital warm air, or pneuma, that sustained life. Galen proposed that vital spirit traveled from the placenta through the umbilical arteries. It was not until 1569 that an anatomist named Aranzio first argued that the maternal and fetal blood systems were separate, rather than continuous. In the 1670s, before oxygen was even discovered, researchers grasped that respiration involved some kind of exchange with the air, though they gave it names like “nitro-aerial particles.” An analogy between placental and pulmonary gas exchange was described as early as 1674 by the English chemist John Mayow. Lavoisier’s identification of oxygen in 1779 finally provided the framework to understand what was really happening, and by 1876 a researcher named Zweifel had proved that fetal blood actually picks up oxygen.20PubMed. Pulmo uterinus: a history of ideas on fetal respiration The fact that it took humanity from Aristotle to the late nineteenth century to work out the basic answer to “how does a baby breathe before birth” says something about how counterintuitive the placental system really is.
Placentation Across the Animal Kingdom
The placenta is not unique to humans, and neither is the problem of getting oxygen to an embryo that cannot yet breathe. All amniote vertebrates, the group that includes mammals, birds, and reptiles, develop a set of fetal membranes. In egg-laying species like birds and most reptiles, the chorioallantoic membrane pressed against the inside of the eggshell serves as the primary gas exchange organ, pulling oxygen in from the outside air and releasing carbon dioxide. In live-bearing species, those same membranes have been repurposed into placentas of wildly varying complexity. Highly developed placentas have evolved independently multiple times, not just in placental mammals like humans but also in certain lizards and even some sharks.21PubMed. Evolution and development of fetal membranes and placentation in amniote vertebrates The underlying challenge, delivering oxygen to a developing organism sealed inside a parent’s body or an egg, is so fundamental that evolution has solved it over and over again with different structural designs built from the same basic toolkit of fetal membranes.