The placenta is the only organ humans grow temporarily, and it single-handedly keeps a fetus alive for roughly nine months. It functions as the baby’s lungs, kidneys, liver, and immune system all at once, exchanging oxygen and nutrients, removing waste, producing hormones, and transferring protective antibodies. No other organ does so many different jobs in such a short lifespan.
How the Placenta Delivers Oxygen
A fetus can’t breathe, so every molecule of oxygen it uses has to cross from the mother’s blood into the fetal blood through the placenta. By late pregnancy, roughly 841 milliliters of blood per minute flows through the uterine vessels that supply the placenta. That’s a remarkable volume, higher per kilogram of fetal weight than in any other mammal studied.
Oxygen moves across the placental barrier by diffusion, driven by the difference in oxygen concentration between maternal and fetal blood. The barrier itself is astonishingly thin: in the smallest branches of the placenta, called terminal villi, the distance between maternal and fetal blood narrows to about 3.7 micrometers. That’s thinner than a single red blood cell. This minimal distance lets gases cross almost instantly.
Fetal blood also has a built-in advantage. Fetal hemoglobin, the protein in red blood cells that carries oxygen, has a slightly different structure than the adult version. It binds oxygen more tightly, essentially pulling oxygen out of the mother’s blood even when concentrations are low. At the same time, a clever chemical exchange happens: as the mother’s blood picks up carbon dioxide from the fetus, it becomes slightly more acidic, which causes it to release even more oxygen. The two processes reinforce each other, maximizing how much oxygen reaches the baby with every pass of blood through the placenta.
Nutrient Transfer and Waste Removal
Oxygen isn’t the only thing crossing the placental barrier. Glucose, amino acids, fatty acids, vitamins, and electrolytes all move from mother to fetus through the same network of tiny villi. These terminal villi look like clusters of grapes under a microscope, and despite their small size, they account for about 50% of the placenta’s total surface area. That enormous surface, packed into an organ that weighs about a pound at full term, is what makes efficient nutrient exchange possible.
Waste removal works in reverse. Carbon dioxide, the main byproduct of fetal metabolism, diffuses back across the placenta into the mother’s bloodstream, where her lungs exhale it. Other metabolic waste products follow the same route. The mother’s kidneys and liver then process these waste products as if they were her own. In this sense, the placenta turns the mother’s organs into a life-support system for the fetus.
Hormones That Sustain Pregnancy
The placenta is also a hormone-producing powerhouse. It churns out estrogen, progesterone, and a hormone called human placental lactogen (hPL), each with distinct roles in keeping the pregnancy on track and reshaping the mother’s metabolism to support fetal growth.
Progesterone and estrogen begin influencing insulin sensitivity as early as six weeks of pregnancy. During the first and second trimesters, these hormones favor fat storage in the mother’s body, building energy reserves. By the third trimester, the metabolic strategy shifts. hPL ramps up fat breakdown, releasing fatty acids into the mother’s blood so that more of her glucose can be diverted to the growing fetus. hPL also stimulates the growth and survival of insulin-producing cells in the mother’s pancreas, helping her body keep up with the increased metabolic demand. This hormonal choreography is what ensures the fetus receives a steady supply of fuel even when the mother hasn’t eaten recently.
Immune Protection Before Birth
The placenta solves one of the most paradoxical problems in biology: the fetus carries half its DNA from its father, making it genetically foreign to the mother’s immune system. Under normal circumstances, the body attacks foreign tissue. The placenta prevents this.
Specialized cells on the placenta’s surface, called trophoblasts, actively train the mother’s immune cells to tolerate the fetus. They release signaling molecules that recruit immune cells to the boundary between mother and fetus and then shift those cells into a tolerant, anti-inflammatory state. Certain white blood cells are converted into regulatory types that suppress the immune attack that would otherwise occur. This isn’t passive shielding. It’s active immune reprogramming that the placenta orchestrates throughout pregnancy.
The placenta also works in the other direction, transferring the mother’s antibodies to the fetus. Starting in the second trimester, a specific type of antibody (IgG) crosses from maternal blood into fetal blood through specialized receptors on the placenta’s surface. These antibodies give the newborn a temporary but critical immune defense against infections the mother has already encountered, whether through illness or vaccination. This borrowed immunity protects the baby during the first vulnerable months of life before its own immune system fully matures.
What Happens When the Placenta Underperforms
Because the placenta handles so many vital functions, problems with it can have serious consequences. The most common issue is placental insufficiency, where blood flow to or through the placenta is reduced. This typically happens when the placenta doesn’t embed deeply enough into the uterine wall during early development. The blood vessels that supply the placenta remain too narrow, restricting flow and reducing the oxygen and nutrients that reach the fetus.
The result is often fetal growth restriction, where the baby grows more slowly than expected. A fetus experiencing chronic low oxygen adapts by conserving energy, which is why reduced fetal movement can be an early sign of placental problems. Blood flow in the baby’s brain increases as the body redirects its limited oxygen supply to protect the most critical organ. If placental function deteriorates significantly, carbon dioxide builds up in fetal blood, the blood becomes more acidic, and the fetus may shift to less efficient forms of energy production that generate lactic acid.
Maternal nutrition also plays a role. Inadequate weight gain before 24 weeks of pregnancy (less than about 4.3 kilograms) is an independent predictor of low birth weight, partly because it reflects insufficient resources for the placenta to transfer.
Delivery of the Placenta
After the baby is born, the placenta still has one final stage. It detaches from the uterine wall and is delivered in what’s known as the third stage of labor, which takes an average of six to seven minutes. A placenta that takes significantly longer to deliver is considered prolonged and raises the risk of excessive bleeding, since the uterus can’t fully contract and seal off the blood vessels at the attachment site until the placenta is out. This is one reason healthcare teams monitor this stage closely, even though the baby has already arrived.