The placenta is a temporary organ that grows alongside your baby during pregnancy, acting as the sole interface between your bloodstream and your baby’s. It delivers oxygen and nutrients, removes waste, produces hormones that sustain the pregnancy, and shields the fetus from infections. Despite being essential to every moment of fetal development, it’s built from scratch in the first trimester and expelled from your body within minutes of birth.
How the Placenta Forms
The placenta begins developing in the earliest days of pregnancy. Between weeks 0 and 13, the fertilized embryo embeds itself into the uterine wall, and placental tissue starts growing from the same cluster of cells that becomes the baby. Specialized cells called trophoblasts burrow into the uterine lining and remodel the mother’s blood vessels, redirecting maternal blood flow into a pool-like space where the placenta can access it.
By the end of the first trimester, the placenta has grown large enough to take over hormone production from the ovaries. Before that point, a small structure in the ovary called the corpus luteum produces the progesterone and estrogen needed to keep the pregnancy going. Once the placenta reaches sufficient mass, it becomes the primary source of both hormones for the remainder of pregnancy.
At full term, the placenta typically weighs about 470 grams, measures around 22 centimeters in diameter, and is roughly 2.5 centimeters thick at the center. For context, a healthy baby usually weighs about five to six times more than its placenta.
The Barrier That Keeps Blood Separate
One of the most common misconceptions about the placenta is that maternal and fetal blood flow together. They don’t. A continuous layer of fused cells called the syncytiotrophoblast separates the two blood supplies. This layer is unusual in biology: rather than being made of individual cells side by side, it’s one giant, multinucleated sheet with no gaps between cells. By the end of pregnancy, this surface spans roughly 12 square meters, about the size of a small room.
Maternal blood flows into open spaces on one side of this barrier, and fetal blood circulates through tiny vessels on the other side. Nutrients and gases pass through the barrier by diffusion or active transport, but the blood itself never crosses over. This design also provides a defense against infection. The fused cell layer has very few surface receptors that pathogens could latch onto, and because there are no junctions between individual cells, microbes can’t slip through the cracks the way they do in other tissues. An unusually dense internal scaffolding of structural proteins reinforces the barrier further, physically resisting the deformations that bacteria and parasites use to push their way into cells.
Delivering Oxygen and Nutrients
Every calorie and every breath of oxygen your baby receives during pregnancy comes through the placenta. Oxygen dissolved in maternal blood diffuses across the placental barrier into fetal blood vessels, much the way oxygen moves across lung tissue. Carbon dioxide travels in the opposite direction, from fetus to mother, where her lungs exhale it.
Glucose, the primary fuel for fetal growth, crosses the placenta through dedicated transport channels embedded in the barrier’s cell membrane. Amino acids, the building blocks of protein, are actively pumped across, meaning the placenta spends energy to move them against a concentration gradient. Fatty acids, vitamins, and minerals each have their own transport pathways. The placenta isn’t a passive filter. It actively selects, concentrates, and regulates what reaches the baby.
Removing Fetal Waste
A developing baby produces metabolic waste just like any other living organism, but its kidneys and liver aren’t mature enough to handle the full job. The placenta fills in. Urea, the main waste product from protein metabolism, crosses back into the mother’s bloodstream by simple diffusion. Bilirubin, a byproduct of red blood cell breakdown, and small amounts of creatinine also transfer to the maternal circulation for disposal. The mother’s kidneys and liver then process these waste products as if they were her own.
Hormones That Sustain Pregnancy
The placenta is one of the most active hormone-producing organs in the human body. Its hormonal output shifts throughout pregnancy to match changing demands.
Human chorionic gonadotropin, or hCG, is the hormone detected by pregnancy tests. Early in pregnancy, hCG signals the corpus luteum in the ovary to keep producing progesterone, preventing menstruation and maintaining the uterine lining. It also helps immature placental cells mature into the hormone-producing syncytiotrophoblast layer, essentially building the factory that will later replace it.
Progesterone and estrogen, initially supplied by the ovary, are produced in increasing quantities by the placenta itself starting late in the first trimester. Progesterone keeps the uterine muscle relaxed to prevent premature contractions and supports the blood vessel changes needed to sustain blood flow to the placenta. Estrogen promotes uterine growth and helps prepare breast tissue for milk production.
In the third trimester, the placenta produces large amounts of human placental lactogen, a hormone that reshapes the mother’s metabolism. It shifts the mother’s energy use away from glucose and toward fat, effectively reserving more glucose for the growing baby. This metabolic shift is one reason gestational diabetes can develop: the hormone makes maternal tissues more resistant to insulin.
Passing Immunity to the Baby
Newborns enter the world with an immune system that has never encountered a pathogen, yet they arrive with a supply of antibodies borrowed from their mother. The placenta actively transports a specific class of antibodies, called IgG, from maternal blood into fetal circulation. This is the only antibody type that crosses the placenta, and it provides the baby with temporary protection against infections the mother has already fought off or been vaccinated against.
The transfer works through a specialized receptor embedded in placental cells. After maternal antibodies are passively absorbed into the cell, the acidic environment inside cellular compartments triggers the receptor to bind tightly to the antibody and shuttle it from the maternal side to the fetal side. This process ramps up as pregnancy progresses, which is one reason premature babies are more vulnerable to infection: they’ve had less time to accumulate their mother’s antibodies. The borrowed protection typically lasts several months after birth, bridging the gap until the infant’s own immune system matures.
What Happens After Birth
Once the baby is delivered, the placenta has completed its job. During the third stage of labor, uterine contractions continue and the placenta detaches from the uterine wall. Most placentas are delivered within 15 minutes. When this stage takes longer than 15 minutes, the risk of excessive bleeding increases roughly fivefold. If it extends beyond 30 minutes, the likelihood of needing a blood transfusion more than triples.
A placenta that hasn’t separated after 30 to 60 minutes is generally considered retained and may need medical intervention. Interestingly, research has not found strong evidence that removing the placenta earlier than it would come on its own actually reduces complications. The uterus typically handles the process effectively when given time, though medical teams monitor closely for signs of heavy bleeding.