The placenta is a temporary organ that keeps a fetus alive by delivering oxygen, removing waste, producing hormones, and transferring immune protection from mother to baby. It forms early in pregnancy, attaches to the uterine wall, and serves as the sole life-support system until birth. No other human organ grows from scratch, performs the jobs of the lungs, liver, kidneys, and endocrine system simultaneously, and then gets discarded within an hour of completing its work.
How the Placenta Delivers Oxygen and Nutrients
The placenta acts as a transfer station between two blood supplies that never actually mix. Only two thin cell layers separate maternal blood from fetal blood. Oxygen crosses this barrier by simple diffusion, driven by the difference in oxygen concentration between the mother’s blood and the baby’s. The barrier is remarkably thin, just 2 to 3 micrometers, which is roughly one-thirtieth the width of a human hair.
Nutrients like amino acids and iron require more active handling. The outer layer of the placenta is studded with specialized transporter proteins that actively shuttle these molecules across. Fatty acids, which are critical for fetal brain development, get released from the mother’s blood lipids by enzymes on the placental surface and then carried across by dedicated fatty acid transporters. The placenta isn’t a passive filter. It selects what to move, how much, and when.
By the end of pregnancy, uterine blood flow reaches nearly 1,000 milliliters per minute, almost double what it was at 20 weeks. That’s close to a liter of blood cycling through the placenta every 60 seconds, ensuring the rapidly growing fetus gets enough of everything it needs in the final stretch.
Waste Removal: The Baby’s Substitute Kidneys and Lungs
A fetus can’t breathe or urinate in any meaningful way to clear waste, so the placenta handles that too. Carbon dioxide diffuses from fetal blood into maternal blood, moving down the same kind of concentration gradient that drives oxygen in the opposite direction. A clever chemical trick makes this exchange more efficient: as the mother’s blood picks up carbon dioxide and becomes slightly more acidic, it releases oxygen more readily. At the same time, the fetal blood, now carrying fresh oxygen, loses its ability to hold carbon dioxide and dumps it back to the mother. This two-way swap happens continuously and automatically.
Other metabolic waste products follow similar routes, crossing the thin placental barrier into the mother’s bloodstream so her kidneys and lungs can dispose of them.
A Hormone Factory That Sustains Pregnancy
The placenta is one of the most active hormone-producing organs in the human body. By the time pregnancy reaches full term, it churns out roughly 250 milligrams of progesterone per day, pushing blood levels to 100 to 200 nanograms per milliliter. Progesterone does several things at once: it maintains the uterine lining so the embryo stays implanted, suppresses the mother’s immune response so her body doesn’t reject the fetus, and shifts immune signaling toward a profile that supports pregnancy rather than fighting it.
The placenta also produces human placental lactogen, a hormone that reshapes the mother’s metabolism. It makes maternal tissues more resistant to insulin, which keeps blood sugar levels slightly elevated and available for the fetus. It also promotes the breakdown of fat stores, increasing circulating fatty acids as another fuel source for the baby. In effect, this hormone redirects the mother’s energy economy to prioritize fetal growth.
Estrogen production requires a collaboration. The placenta lacks certain enzymes needed to build estrogens from scratch, so it relies on a precursor compound made by the fetal adrenal glands. The placenta then converts this precursor into active estrogen. It’s one of the clearest examples of how the placenta and the fetus work as a biochemical team.
Immune Protection Before Birth
Newborns enter the world with immature immune systems, but they aren’t defenseless. The placenta actively transports the mother’s antibodies into fetal circulation, giving the baby a starter kit of immune protection that lasts for the first months of life.
Only one class of antibody, IgG, crosses the placenta in significant amounts. This transfer begins around week 13 of pregnancy but starts small. At weeks 17 to 22, fetal antibody levels are only 5 to 10 percent of the mother’s. By weeks 28 to 32, they’ve climbed to about 50 percent. Then a sharp increase occurs after week 36, and by full term, the baby’s IgG levels actually exceed the mother’s by 20 to 30 percent. This timing explains one reason premature babies face higher infection risk: they miss the final surge of antibody transfer.
The placenta uses a specific receptor on its surface to grab IgG molecules from maternal blood and shuttle them across to the fetus. The same receptor, unfortunately, can occasionally be exploited by certain viruses, including Zika and HIV, as a route into the placenta.
A Selective Barrier Against Infection
While the placenta transfers nutrients and antibodies, it simultaneously acts as a physical shield against most pathogens. The continuous outer cell layer that lines the placenta has no gaps or cell junctions for bacteria to slip through, which is why most common infections a pregnant person experiences never reach the fetus.
The barrier isn’t perfect. A small number of pathogens have evolved ways to breach it. Viruses like cytomegalovirus, Zika, and rubella are among the most commonly transmitted infections across the placental barrier. Certain parasites (the organism that causes toxoplasmosis) and bacteria (Listeria) can cross through cell-to-cell transmission. The malaria parasite takes a different approach entirely, binding to a molecule on the placental surface and causing local inflammation and damage that can compromise the barrier.
The placenta also expresses iron transporters on its surface to supply the fetus with iron. Some viruses, including hepatitis C, may hijack these iron transport pathways to gain entry. So while the placenta blocks the vast majority of threats, it has inherent vulnerabilities wherever it has receptors designed to move essential substances across.
How and When the Placenta Forms
Placental development begins in the first days after a fertilized egg implants in the uterine wall, typically within the first two weeks of pregnancy. During the first 13 weeks, specialized cells from the embryo invade the uterine wall and begin remodeling the mother’s blood vessels, converting them into wide, low-resistance channels that can deliver large volumes of blood to the developing placenta. During this early phase, the placenta operates in a relatively low-oxygen environment.
As pregnancy progresses, the placenta develops an increasingly dense network of branching finger-like projections called villi, which maximize the surface area available for exchange. By the third trimester, this branching network is extensive enough to support the enormous metabolic demands of a near-term fetus. The entire organ is typically fully functional by around 12 weeks but continues to grow and adapt throughout pregnancy.
Medical Uses After Delivery
Placental tissue has genuine medical applications beyond pregnancy. Placental mesenchymal stem cells, a type of stem cell harvested from donated placentas, are being studied in clinical trials for wound healing. A recent phase II trial found that a hydrogel containing these stem cells significantly accelerated recovery from radiation-induced skin damage in cancer patients, reduced pain, and prevented wounds from expanding. Early-phase trials are also testing placental stem cells for diabetic foot ulcers.
These applications take advantage of the placenta’s natural regenerative properties and its immune-tolerant biology, which makes placental cells less likely to trigger rejection when used in other people.
Eating the Placenta: What the Evidence Shows
Placentophagy, the practice of consuming one’s own placenta after birth (typically dried and put into capsules), has gained popularity based on claims that it boosts breast milk production, reduces postpartum depression, improves energy, and replaces lost nutrients. The scientific evidence does not support these claims.
Controlled studies have found that the encapsulation process destroys many of the nutrients and active compounds in fresh placental tissue, making it unlikely that capsules deliver any meaningful physiological effect. Researchers who have reviewed the available data conclude that the reported benefits are most likely a placebo effect. The Society of Obstetricians and Gynecologists of Canada explicitly does not recommend the practice, citing a lack of methodologically sound studies.
The risks, on the other hand, are more concrete. Placental capsules can contain harmful levels of heavy metals like arsenic, lead, cadmium, and mercury. They can harbor bacteria, including Group B Streptococcus, which poses a real danger to newborns. Placentas from mothers who smoke may contain elevated cadmium. Residues from anesthetics used during labor can also end up in the processed tissue. Because placental capsules are not regulated as food or medicine, there are no safety standards governing their preparation.