The placenta is a temporary organ that grows inside the uterus during pregnancy, connecting the developing fetus to the mother’s blood supply. It handles an extraordinary range of jobs: ferrying oxygen and nutrients to the fetus, producing hormones that reshape the mother’s metabolism, shielding the fetus from immune attack, and acting as a selective filter against many harmful substances. No other human organ is built from scratch, performs so many distinct functions simultaneously, and is then discarded within hours of completing its work.
How the Placenta Forms
Placental development begins within the first week or so after a fertilized egg implants in the uterine lining. The outer cell layer of the early embryo, called the trophoblast, pushes into the tissue of the uterus and starts building the structures that will eventually become the placenta’s working surface. Within the first two weeks, an inner layer of rapidly dividing cells generates the initial finger-like projections (villi) that reach into the uterine wall. These villi are the placenta’s basic functional units, and they develop a core of connective tissue that is soon invaded by the beginnings of a blood vessel network. The first fetal capillaries appear around days 18 to 20, and specialized immune cells called Hofbauer cells show up in the tissue at roughly the same time, supporting early blood vessel formation.1Human Reproduction Update. Placental formation in early pregnancy: how is the centre of the placenta made?
By about day 32, the placenta’s blood vessels connect to the embryo’s own developing circulatory system through the umbilical cord. At this point, the mature architecture of the villi is taking shape: fetal blood vessels and immune cells sit within a loose core, surrounded by a double layer of trophoblast cells. The outer layer of this shell is a single continuous sheet of fused cells called the syncytiotrophoblast, which covers the entire surface of the placenta and is the actual interface where nutrient and gas exchange happens. Cells at the tips of anchoring villi take a different path, burrowing into the uterine wall and remodeling the mother’s blood vessels to increase blood flow to the placenta.1Human Reproduction Update. Placental formation in early pregnancy: how is the centre of the placenta made?
Feeding and Breathing for Two
The placenta’s most intuitive role is as a supply line. The fetus cannot eat, drink, or breathe on its own, so every molecule of oxygen and every nutrient it needs must cross from the mother’s bloodstream, through the placental tissue, and into the fetal circulation. This transfer depends on specialized transport proteins embedded in the syncytiotrophoblast. Different nutrients rely on different transporters: glucose, amino acids, fatty acids, and minerals each have their own molecular machinery to move them across the placental barrier.
How well these transporters work directly influences how fast the fetus grows. When the expression or activity of specific transporters changes, fetal growth can be restricted or excessive. Researchers now view the placenta not as a passive conduit but as an active gatekeeper that adjusts nutrient flow based on what is available in the mother’s blood, almost like a control system trying to match fetal demand to maternal supply.2PubMed Central. Maternal-fetal nutrient transport in pregnancy pathologies: the role of the placenta
Gas exchange works on a simpler principle. Oxygen diffuses from the mother’s blood, where its concentration is higher, into fetal blood, where it is lower. Carbon dioxide moves the other way. The efficiency of this process depends heavily on how well the villi are vascularized and how thin the tissue layer is between the two bloodstreams. By late pregnancy, the barrier between maternal and fetal blood in some regions of the placenta is only a few micrometers thick.
A Hormone Factory
The placenta is one of the most prolific hormone-producing organs in the body, and the hormones it releases reshape nearly every aspect of the mother’s physiology. It produces steroid hormones like progesterone and estrogen, protein hormones like human chorionic gonadotropin (hCG, the hormone pregnancy tests detect), and a wide array of growth factors and signaling molecules.3PubMed Central. Placental Endocrine Activity: Adaptation and Disruption of Maternal Glucose Metabolism in Pregnancy and the Influence of Fetal Sex
One major effect of placental hormones is on the mother’s metabolism. As pregnancy progresses, the placenta produces hormones that make the mother’s tissues less responsive to insulin. This shifts her metabolism so that more glucose stays available in the blood for the fetus to use. In most pregnancies, the mother’s pancreas compensates by producing more insulin. When this compensation fails, the result is gestational diabetes. The placenta also produces hormones that increase maternal blood volume, relax blood vessel walls, and suppress uterine contractions until labor. It is, in effect, running the biochemical program that keeps pregnancy going.
Hiding the Fetus From the Immune System
Half of a fetus’s genes come from the father, which means fetal tissue carries proteins the mother’s immune system has never seen before. In any other context, those foreign proteins would trigger an immune attack. The placenta prevents this through multiple overlapping strategies, not a single trick.
The physical barrier of the syncytiotrophoblast is the first line of defense: it sits between the mother’s immune cells and the fetal tissue, limiting direct contact. Beyond the physical barrier, the placenta restricts which immune cells can migrate into the area, expresses molecules that act as “off switches” for immune activation, and promotes the expansion of specialized regulatory immune cells that actively suppress inflammatory responses.4PubMed Central. Multi-Layered Mechanisms of Immunological Tolerance at the Maternal-Fetal Interface Even the sugar molecules decorating the surface of placental cells play a role, conferring a kind of immune camouflage.
This system is remarkably effective. Outright immune rejection of a fetus is rare. When pregnancies do fail, the cause is more often inflammation at the placental interface than a classical immune rejection response.5PubMed Central. Tolerance of the fetus by the maternal immune system: role of inflammatory mediators at the feto-maternal interface
Remodeling the Mother’s Blood Vessels
For the placenta to receive enough blood, it cannot simply tap into the mother’s existing circulation. Specialized trophoblast cells invade the walls of the uterine spiral arteries and transform them from narrow, muscular vessels into wide, low-resistance channels that pour blood freely into the spaces around the placental villi.6PubMed Central. Human trophoblast invasion and spiral artery transformation: the role of PECAM-1 in normal pregnancy, preeclampsia, and fetal growth restriction This remodeling is one of the most aggressive things any human tissue does: the trophoblast cells essentially replace the normal lining and muscle of these arteries, turning them into passive conduits.
When this process goes wrong, the consequences can be severe. Inadequate spiral artery remodeling is at the center of preeclampsia, a dangerous condition marked by high blood pressure, protein in the urine, and organ damage in the mother. The poorly remodeled arteries deliver insufficient blood to the placenta, which becomes oxygen-starved. In response, the ischemic placenta releases large amounts of a protein called sFlt-1, which mops up growth factors that the mother’s blood vessels need to stay healthy. The result is widespread damage to the mother’s blood vessel lining.7PubMed Central. Pathogenesis of Preeclampsia and Therapeutic Approaches Targeting the Placenta There is also evidence that excess sFlt-1 may play a role in deranging the placenta’s own development, creating a vicious cycle.8JCI Insight. Excess placental soluble fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial dysfunction, hypertension, and proteinuria in preeclampsia
The Placenta as a Selective Filter
For decades, the placenta was assumed to be a robust barrier that kept harmful substances away from the fetus. The thalidomide disaster of the late 1950s and 1960s shattered that assumption, revealing that many drugs could cross the placenta and cause devastating birth defects.9PubMed. The role of the placenta in drug transport and fetal drug exposure Since then, the picture has become more nuanced. The placenta does have active defense mechanisms, but they are selective rather than absolute.
The syncytiotrophoblast expresses a family of efflux transporters, proteins that actively pump certain molecules back out toward the maternal blood before they can reach the fetus. These transporters recognize and expel a range of drugs and environmental chemicals.10PubMed Central. Drug transporters in the human blood-placental barrier But many small, fat-soluble molecules slip through easily, which is why alcohol, nicotine, and certain medications reach the fetus with little resistance. The placenta’s filtering ability varies by substance, by stage of pregnancy, and to some extent by individual variation in transporter expression.
When the Placenta Underperforms
Placental insufficiency, a broad term for when the placenta cannot deliver enough oxygen and nutrients to meet fetal needs, is one of the leading causes of fetal growth restriction. In severe cases, the blood vessels within the placental villi are abnormally thin and elongated, creating high resistance to blood flow. This resistance shows up on ultrasound as abnormal patterns in the umbilical artery, and it is associated with significantly increased risks of complications for the fetus.11PubMed Central. Impaired fetoplacental angiogenesis in growth-restricted fetuses with abnormal umbilical artery doppler velocimetry is mediated by aryl hydrocarbon receptor nuclear translocator (ARNT)
In a study of pregnancies with severe placental insufficiency detected before the fetus reached viability, the outcomes depended heavily on whether blood flow patterns improved over time. Fetuses whose Doppler readings normalized were delivered later and at higher birth weights, while those with persistently abnormal flow faced delivery around 28 weeks and much higher complication rates.12PubMed. Outcome of severe placental insufficiency with abnormal umbilical artery Doppler prior to fetal viability There is no treatment that directly fixes a poorly functioning placenta. Management revolves around monitoring, timing delivery to balance the risks of prematurity against the risks of a failing placental supply, and in some cases administering steroids to accelerate fetal lung development before an early delivery.
Placenta Accreta Spectrum
On the opposite end of the invasion spectrum from shallow implantation is placenta accreta, where trophoblast cells burrow too deeply. In normal pregnancies, a layer of tissue called the decidua acts as a boundary, preventing the placenta from growing into the muscular wall of the uterus. In placenta accreta spectrum disorders, this boundary is defective, and trophoblast cells invade into the muscle layer or, in the most severe cases, through the uterine wall entirely.
The main risk factor is a prior cesarean section. The scar left behind provides a site where the decidual barrier is thin or absent. When a subsequent pregnancy implants at or near that scar, the trophoblast cells encounter little resistance and invade abnormally deep. The risk rises with each additional cesarean delivery.13PubMed. Etiopathogenesis and risk factors for placental accreta spectrum disorders As the high-pressure maternal blood supply feeds directly into these abnormally deep tissues, it creates large blood-filled spaces (lacunae) that can cause life-threatening hemorrhage at delivery.14Clinical Obstetrics and Gynecology. Pathophysiology of Placenta Accreta Spectrum Disorders: A Review of Current Findings With cesarean rates climbing worldwide, placenta accreta spectrum disorders have become substantially more common in recent decades.
Fetal Cells That Stick Around
One of the stranger consequences of having a placenta is microchimerism: the two-way exchange of cells between mother and fetus. During pregnancy, small numbers of fetal cells cross the placenta into the mother’s bloodstream, and a smaller number of maternal cells cross the other way. These foreign cells can persist in the host for decades after pregnancy ends.15PubMed. Fetomaternal microchimerism in tissue repair and tumor development
The implications are still being worked out. Fetal cells have been found in maternal tissues including the thyroid, liver, brain, and skin. There is evidence that some of these cells may participate in tissue repair, and researchers are investigating possible links to autoimmune diseases and even certain cancers, though these connections remain uncertain. What is clear is that pregnancy leaves a lasting cellular trace in both mother and child, a biological signature that outlives the placenta itself by years or decades.16Trends in Molecular Medicine. Biology and clinical relevance of fetal–maternal cell trafficking and microchimerism
The Placental Microbiome Debate
Around 2014, a widely discussed study suggested that the placenta harbors a resident community of bacteria, a “placental microbiome.” This challenged the long-held assumption that the womb is sterile and prompted a wave of research. The idea was exciting: if the fetus is exposed to bacteria before birth, the implications for immune development and infant health would be profound.
Subsequent work has largely deflated the claim. A critical assessment of the original studies pointed out that they used DNA-based detection methods with insufficient sensitivity for the very low levels of bacteria they claimed to find, lacked proper controls for contamination (which is rampant in molecular biology reagents), and did not demonstrate that any bacteria were actually alive.17PubMed Central. A critical assessment of the “sterile womb” and “in utero colonization” hypotheses: implications for research on the pioneer infant microbiome A carefully controlled study of placentas from full-term pregnancies found that when you measure bacterial DNA in placental tissue and compare it to blank laboratory controls, there is no meaningful difference: the bacteria detected are coming from the reagents and lab environment, not the placenta.18PubMed. No evidence for a placental microbiome in human pregnancies at term The current consensus leans toward the traditional view: in a healthy pregnancy, the placenta is sterile or very nearly so.
Microplastics Crossing the Barrier
A more recent and less reassuring discovery is that microplastics can be found in human placental tissue. A 2020 study examining six placentas found microplastic fragments ranging from 5 to 10 micrometers in four of them, located on both the fetal and maternal sides as well as in the surrounding membranes.19PubMed. Plasticenta: First evidence of microplastics in human placenta A systematic review confirmed the finding across multiple studies, with detected particle sizes ranging from about 2 to 100 micrometers, and evidence from fetal meconium (the first stool) suggesting that at least some particles cross the placenta and reach the fetus.20PubMed Central. Impact of Microplastics on Pregnancy and Fetal Development: A Systematic Review
What these particles actually do to developing fetal tissue is still unknown. Microplastics can carry surface contaminants and may trigger low-level inflammation, but no studies have yet established a clear link between placental microplastic levels and specific pregnancy complications. The research is in its earliest stages, and the field is grappling with basic methodological challenges like standardizing how particles are measured and classified.
Programming Future Health
The placenta does more than sustain pregnancy in real time. It also shapes the fetus’s long-term health trajectory through a process researchers call developmental programming. The idea, sometimes referred to as the developmental origins of health and disease (DOHaD) hypothesis, is that conditions during fetal life can alter gene expression patterns that persist into childhood and adulthood, affecting risks for heart disease, diabetes, obesity, and other conditions.
The placenta sits at the center of this process because it mediates every environmental signal the fetus receives. Maternal smoking, air pollution, heavy metal exposure, and nutritional status can all leave chemical marks on placental DNA (epigenetic modifications) that change how genes are read without altering the genetic code itself. Placental DNA methylation and RNA expression patterns have been linked to prenatal exposures including particulate air pollution, metals, and smoking, as well as to infant outcomes like birth weight and early neurobehavior.21PubMed Central. The Placental Epigenome as a Molecular Link Between Prenatal Exposures and Fetal Health Outcomes Through the DOHaD Hypothesis These effects can even differ by the sex of the fetus, with male and female placentas sometimes responding differently to the same environmental stress.22PubMed Central. Placental contribution to the origins of sexual dimorphism in health and diseases: sex chromosomes and epigenetics
Because the placenta is discarded after birth, it offers a unique window into what happened during pregnancy. Researchers can analyze placental tissue to reconstruct the fetal environment and look for epigenetic signatures associated with later health problems, effectively using the placenta as a biological record of prenatal life.23PubMed Central. Placental Epigenetics in Children’s Environmental Health
Placentas Beyond Mammals
The word “placenta” is so strongly associated with mammals that most people are surprised to learn placentas have evolved independently many times across the animal kingdom. Sharks, bony fishes, coelacanths, amphibians, and squamate reptiles (lizards and snakes) all include species that have developed some form of placenta, defined broadly as a close association of fetal and parental tissue for physiological exchange.24PubMed Central. Embryonic specializations for vertebrate placentation These independent origins mean that nature has repeatedly converged on the same basic solution: bring parent and offspring tissue into close contact so resources can be transferred directly.
Even among mammals, placental structure varies enormously. Placentas are classified by how many tissue layers separate the mother’s blood from the fetus’s. Humans, rodents, and higher primates have hemochorial placentas, where maternal blood directly bathes the trophoblast surface. Dogs and cats have endotheliochorial placentas with an extra tissue layer, while horses, pigs, and whales have epitheliochorial placentas with the most layers of separation. These structural differences do not track neatly with evolutionary relatedness; some closely related groups have very different placental types.25PubMed Central. Comparative aspects of trophoblast development and placentation Part of the explanation may involve endogenous retroviruses, ancient viral sequences embedded in mammalian DNA that have been co-opted over millions of years to drive the formation and diversification of placental tissues.26PubMed Central. Endogenous Retroviruses and Placental Evolution, Development, and Diversity
Studying the Placenta Without a Pregnancy
The placenta is one of the hardest human organs to study. You cannot biopsy it during a healthy pregnancy without risk, and by the time it is delivered, the pregnancy is already over. Most of what we know comes from examining placentas after birth or from animal models, neither of which fully captures how a living human placenta functions in real time.
A newer approach uses placenta-on-a-chip technology: microfluidic devices that recreate the layered structure of the placental barrier on a tiny scale, with trophoblast cells on one side and endothelial cells on the other, separated by a membrane, with fluid flowing on both sides to simulate blood circulation. These models have been used to study glucose transfer, drug transport, the behavior of efflux pumps, and even pathological conditions like bacterial infection and preeclampsia.27PubMed Central. Placenta-on-a-Chip as an In Vitro Approach to Evaluate the Physiological and Structural Characteristics of the Human Placental Barrier upon Drug Exposure: A Systematic Review Another line of research has developed organoid models from trophoblast stem cells, creating miniature barrier structures that replicate the selectivity of the real placenta closely enough to block large molecules from crossing while allowing smaller ones through.28Nature Communications. Trophoblast stem cell-based organoid models of the human placental barrier
These tools matter because the safety testing of medications during pregnancy has historically relied on imperfect proxies. Pregnant people are almost always excluded from drug trials for ethical reasons, leaving doctors to prescribe based on limited data. Lab-on-a-chip and organoid models offer a way to screen how drugs and environmental exposures interact with placental tissue without putting anyone at risk, and they are already producing insights that older cell-culture methods could not.
Eating the Placenta
Placentophagy, the practice of consuming the placenta after birth, is common in many non-human mammals and has seen a resurgence in popularity among humans, often in the form of dehydrated placenta capsules. Proponents claim benefits including increased breast milk production, reduced postpartum depression, improved mood, and faster recovery. The evidence behind these claims is thin. Reviews of the available research have found that the reported benefits rely heavily on self-reported outcomes without rigorous controls, while the potential risks, including bacterial or viral contamination and exposure to concentrated hormones or trace elements that could be harmful to mother and infant, are more concrete.29PubMed Central. Consumption of Maternal Placenta in Humans and Nonhuman Mammals: Beneficial and Adverse Effects No major medical organization currently recommends the practice, and the CDC has issued at least one warning after a newborn developed a bacterial infection linked to contaminated placenta capsules consumed by the mother.