Yes, the placenta is an organ. It is the only temporary organ the human body produces, forming inside the uterus during pregnancy and being expelled after the baby is born. What makes the placenta unusual among organs is that it develops from scratch, performs the work of multiple organ systems simultaneously, and has a functional lifespan of roughly nine months.
What Makes the Placenta an Organ
An organ is a self-contained structure made of multiple tissue types that performs specific functions for the body. The placenta checks every box. It contains blood vessels, connective tissue, and specialized cells arranged in a defined structure, and it carries out essential functions no other organ replicates during pregnancy. It begins forming shortly after a fertilized egg implants in the uterine lining and shares the same genetic makeup as the fetus, not the mother.
The placenta is specifically classified as an endocrine organ because it produces hormones, but its job description extends far beyond that. It functions as the fetus’s lungs, kidneys, liver, and immune shield all at once. No other single organ in the body serves that many roles simultaneously, which is part of why researchers have historically called it one of the least understood organs in medicine.
How It Handles Gas and Nutrient Exchange
The placenta acts as a go-between for the maternal and fetal blood supplies, which never actually mix. Oxygen and nutrients pass from the mother’s blood through thin layers of placental tissue and into the fetal blood vessels, while carbon dioxide and waste products travel in the opposite direction. The system works because fetal blood has a higher affinity for oxygen than maternal blood. At the same oxygen pressure, fetal blood reaches about 80% oxygen saturation while maternal blood sits at around 50%, effectively pulling oxygen across the barrier.
The placenta is also selective about what it lets through. It allows nutrients and oxygen to pass freely but blocks many toxic compounds from reaching the fetus. This barrier function isn’t perfect, which is why certain medications, alcohol, and infections can still cross, but it provides a meaningful layer of chemical protection throughout pregnancy.
The Hormones It Produces
The placenta manufactures an impressive range of hormones that keep pregnancy viable and prepare the mother’s body for delivery. Early in pregnancy, it produces hCG (the hormone pregnancy tests detect), which signals the ovaries to keep making progesterone. Without this signal, the pregnancy would fail within the first few weeks.
By the end of the first trimester, the placenta grows large enough to take over progesterone and estrogen production on its own. Studies from as early as the 1960s showed that removing both ovaries between weeks 7 and 10 of pregnancy had little impact on the fetus or hormone levels, because the placenta had already assumed control. Later in pregnancy, it produces large amounts of a hormone called human placental lactogen, which helps regulate the mother’s metabolism of fats and sugars to ensure adequate fuel reaches the fetus. It also produces a protein that helps control calcium transport from mother to fetus, and relaxin, which helps modify connective tissue as the body prepares for labor.
Collectively, these hormones regulate the placenta’s own growth, direct fetal development, adjust the mother’s cardiovascular and immune systems, and prepare the uterus for delivery.
How It Protects the Fetus From the Immune System
The fetus carries DNA from both parents, which means half its genetic material is foreign to the mother’s body. Under normal circumstances, the immune system would attack foreign tissue the way it rejects a transplanted organ. The placenta prevents this through a sophisticated system of immune suppression at the point where maternal and fetal tissues meet.
Placental cells display a special molecule called HLA-G on their surface. Unlike the markers found on most cells in the body, HLA-G signals immune cells to stand down rather than attack. The immune cells in the uterine lining, particularly a specialized type of natural killer cell, respond to HLA-G by shifting into a tolerant, non-destructive state. Additional molecular signals at the placental surface use the same “checkpoint” pathways that cancer researchers have studied extensively, including PD-1 and CTLA-4 pathways, to suppress immune activation. The result is a localized zone of immune tolerance that protects the fetus without shutting down the mother’s ability to fight infections elsewhere in her body.
A Chimeric Structure
One of the placenta’s most unusual features is that it contains cells from both the mother and the fetus. The fetal side consists of specialized cells called trophoblasts, which form the outer layer and do most of the hormone production and nutrient transfer. The maternal side consists of modified uterine lining cells that have undergone a transformation called decidualization to support the pregnancy. Researchers using single-cell analysis techniques have been able to separate and individually study these maternal and fetal cell populations from the same placental tissue, revealing that structural cells once thought to be passive scaffolding are active participants in maintaining pregnancy.
It May Not Be Sterile
For decades, the placenta was assumed to be completely sterile, with the fetus developing in a germ-free environment. Advances in microbial detection have challenged that view. Researchers have identified low levels of microorganisms in placental tissue, predominantly non-harmful strains. Interestingly, the microbial community found in the placenta more closely resembles the bacteria in the mouth than those in the gut or birth canal, suggesting that oral and intestinal bacteria may seed the placental microbiome through the bloodstream. This is still a relatively new area of investigation, and the functional significance of these microorganisms is not fully established.
What Happens When It Goes Wrong
Because the placenta performs so many critical functions, problems with it can create serious complications. In placenta previa, the placenta attaches too low in the uterus and partially or fully covers the cervix, which can cause bleeding and complicate delivery. Placental abruption occurs when the placenta separates from the uterine wall before delivery, cutting off the baby’s supply of oxygen and nutrients.
Placenta accreta spectrum describes a range of conditions where the placenta grows too deeply into the uterine wall. In the mildest form, it simply adheres too firmly. In more severe forms, it can invade through the uterine muscle or even penetrate through to surrounding organs. More than 80% of accreta cases occur alongside placenta previa. Ultrasound is the primary tool for detecting these conditions, though its reliability varies. In blinded studies, expert reviewers showed only moderate agreement when interpreting ultrasound images for accreta, with individual accuracy ranging from about 53% to 74% sensitivity.
How the Placenta Is Delivered
After the baby is born, the placenta still needs to come out. This third stage of labor typically takes 6 to 10 minutes, occurring after two or three final uterine contractions. The uterine muscle contracts and shrinks, reducing the surface area where the placenta was attached. Because the placenta is more rigid than the elastic uterine wall, this shrinking creates a shearing force that peels the placenta away, usually starting from the bottom edge and spreading upward.
Once the placenta separates and is expelled, the blood vessels that fed it pass through a lattice of crisscrossing muscle fibers in the uterine wall. As the muscle contracts and shortens, these fibers kink and compress the blood vessels, naturally controlling bleeding. If the placenta hasn’t delivered within 30 minutes, the risk of complications, particularly heavy bleeding, increases significantly.