Your placenta is a temporary organ that grows inside your uterus during pregnancy, connecting you to your developing baby through the umbilical cord. It acts as a lifeline, delivering oxygen and nutrients from your blood to your baby’s blood while keeping the two blood supplies separate. By the time you deliver, it weighs about 510 grams (just over a pound), measures roughly 20 centimeters across, and looks like a thick, spongy disc about 3 centimeters deep.
How the Placenta Works
The placenta’s core job is exchange. Your baby needs oxygen, sugar, amino acids, calcium, and dozens of other nutrients to grow, but your blood and your baby’s blood never actually mix. Instead, maternal blood pools into open spaces on the outer surface of the placenta, and nutrients pass through a thin, multi-layered barrier into tiny fetal blood vessels on the other side. Waste products like carbon dioxide travel the opposite direction, from your baby back into your bloodstream so your body can dispose of them.
The barrier that keeps the two blood supplies apart is made of a continuous layer of fused cells called the syncytiotrophoblast. Because these cells have no gaps between them, they form a true seal. Underneath sits a layer of supporting cells, connective tissue, and the walls of your baby’s capillaries. Together, these layers are selective: oxygen and carbon dioxide slip through easily based on blood flow, glucose crosses by facilitated diffusion, calcium gets actively pumped across, and amino acids are transported using energy from ion gradients. The placenta doesn’t just let things passively leak through. It controls what gets in and what stays out.
A Hormone Factory
Beyond nutrient delivery, the placenta is one of the most active hormone-producing organs in the body. Early in pregnancy, it releases human chorionic gonadotropin (hCG), the hormone that pregnancy tests detect. hCG’s primary job is to keep the corpus luteum in your ovary producing progesterone until the placenta can take over that role itself.
Once the placenta matures, it produces large amounts of progesterone, which keeps the uterine lining stable, prevents contractions, and helps suppress immune responses that might otherwise reject the pregnancy. It also produces estrogens, particularly estriol, which dramatically increases blood flow to the uterus and placenta to support the growing baby.
Two other placental hormones reshape your metabolism. Human placental lactogen shifts your body toward using fat for energy while directing more glucose and other nutrients to the fetus. Placental growth hormone does something similar, stimulating your liver to produce glucose and break down fats to keep nutrient supply high. These metabolic shifts are a big reason pregnant people experience changes in blood sugar regulation, and they contribute to the insulin resistance that can develop in later pregnancy.
Where It Attaches
The placenta can implant in several positions inside the uterus, and your provider will note its location during ultrasound scans. The most common positions include posterior (toward your spine), anterior (toward your belly), fundal (at the top of the uterus), and lateral (on the left or right side). All of these are normal.
An anterior placenta, sitting between your baby and your abdominal wall, acts like a cushion. Most people start feeling kicks around 18 weeks, but with an anterior placenta you may not notice movement until after 20 weeks, and those early kicks can feel softer or more muffled. This doesn’t affect your baby’s health at all.
A low-lying placenta, one that sits near or covers the cervix, is more significant. Early in pregnancy this is common and often resolves on its own as the uterus grows. When it persists into the third trimester, covering part or all of the cervix, it’s called placenta previa, which can cause painless bleeding and typically requires a cesarean delivery.
Complications to Be Aware Of
Most placentas function without problems, but a few conditions can develop. Placental abruption occurs when the placenta separates from the uterine wall before delivery. Symptoms can include abdominal pain, uterine tenderness, and vaginal bleeding, though in some cases bleeding is absent because blood gets trapped behind the placenta. There is no single test that definitively diagnoses abruption. Providers rely on symptoms, physical exam, and ultrasound to rule out other causes like previa.
Placenta accreta is a condition where the placenta grows too deeply into the uterine wall, making it difficult or impossible to separate naturally after birth. This is more common in people who have had prior cesarean deliveries. It’s usually diagnosed by ultrasound before delivery and requires careful surgical planning.
Delivering the Placenta
After your baby is born, you still have one more stage of labor. The third stage is the time between delivering your baby and delivering the placenta. This typically takes about 6 minutes, with most placentas arriving within 4 to 10 minutes after a few additional uterine contractions. If the placenta hasn’t delivered within 30 minutes, the risk of complications like excessive bleeding increases, and your provider will intervene.
At that point, the placenta’s work is done. Your provider will examine it to make sure it came out completely, since retained fragments can cause bleeding or infection.
Cord Blood After Delivery
The blood remaining in the umbilical cord and placenta after birth contains stem cells that can form new blood cells. These stem cells have been used to treat certain blood cancers, inherited immune disorders, and metabolic conditions through transplantation. You can choose to donate cord blood to a public bank for use by unrelated patients, or store it in a private bank for potential future use by your family. Public donation is regulated by the FDA as a biological product, while private storage for personal or family use follows a different set of requirements, including infectious disease screening and proper handling standards.