The placenta is a temporary organ that grows inside the uterus during pregnancy, connecting the developing baby to the mother’s blood supply. It acts as the baby’s lungs, kidneys, liver, and immune system all in one, delivering oxygen and nutrients while removing waste. At full term, the average placenta weighs about 510 grams (just over a pound), measures roughly 18.5 centimeters across, and is about 2.3 centimeters thick.
How the Placenta Forms
The placenta begins developing in the earliest days of pregnancy. After a fertilized egg implants in the uterine wall, specialized cells on its outer surface start burrowing into the uterine lining. These cells eventually form tiny finger-like projections called villi, which extend into the uterine wall and create a massive surface area for exchange between mother and baby. This process starts within the first 13 weeks of pregnancy.
The organ is built from both fetal and maternal tissue. The fetal side produces the branching villi, which are bathed in pools of the mother’s blood. The maternal side is the modified lining of the uterus that supplies that blood. Despite this close contact, the two blood supplies never actually mix. They’re separated by an incredibly thin barrier, just 2 to 3 micrometers thick in some places, that allows substances to pass between them.
How Oxygen and Nutrients Reach the Baby
Oxygen moves from the mother’s blood to the baby’s blood by diffusing across the placental barrier, driven by the difference in oxygen levels on each side. This transfer is remarkably efficient for several reasons. Fetal blood has a different type of hemoglobin (the protein that carries oxygen in red blood cells) that grips oxygen more tightly than the adult version. At the same time, a clever chemical trade-off happens: as the mother’s blood picks up carbon dioxide from the baby, it becomes slightly more acidic, which causes it to release even more oxygen. The baby’s blood does the reverse, dropping off carbon dioxide as it loads up on oxygen.
Nutrients cross the barrier through several routes depending on their size and chemistry. Small molecules pass through easily by passive diffusion. Larger nutrients like glucose and amino acids are actively transported by specialized receptors on the placental cells, ensuring the baby gets what it needs even when concentrations are low.
The Placenta as a Hormone Factory
Beyond its exchange functions, the placenta is one of the most active hormone-producing organs in the body. It manufactures several hormones that are essential for maintaining pregnancy and preparing the body for birth and breastfeeding.
- Human chorionic gonadotropin (hCG) is the hormone detected by pregnancy tests. It maintains the structure in the ovary that produces progesterone early in pregnancy and stimulates the placenta itself to eventually take over hormone production.
- Progesterone keeps the uterine lining thick and receptive, relaxes blood vessels, and slows digestion (which is why constipation is so common in pregnancy).
- Estrogen increases blood flow throughout the body, contributes to skin changes like darkening pigmentation, and helps prepare the breasts for milk production. It’s also behind the nasal congestion and bleeding gums many pregnant people experience.
- Human placental lactogen adjusts the mother’s metabolism to increase the amount of glucose available to the baby while also preparing breast tissue for lactation.
- Relaxin loosens ligaments and joints throughout the body and helps soften the cervix in preparation for delivery.
What the Barrier Blocks and What Gets Through
The placenta is selective, but it’s not a perfect filter. Small, fat-soluble molecules cross easily by slipping through the cell membranes of placental tissue. Most prescription and over-the-counter drugs fall into this category, which is why medication use during pregnancy requires careful consideration. Alcohol, nicotine, and caffeine also cross readily.
Larger molecules have a harder time. Research shows that as molecular weight increases, the ability to cross the placental barrier drops significantly. Very large molecules are essentially blocked entirely, unable to reach the fetal blood supply even after hours of exposure.
There’s one important exception to the size rule. The placenta actively transports one class of antibody, called IgG, from the mother’s blood to the baby’s. IgG is a large molecule, but specialized receptors on the placental cells grab it, pull it through, and release it on the fetal side. This is how newborns arrive with temporary immunity to many of the infections their mothers have fought off or been vaccinated against. The amount transferred depends on the mother’s antibody levels, the stage of pregnancy, and the health of the placenta itself.
Waste Removal
The placenta functions as the baby’s kidneys and lungs in reverse. Carbon dioxide produced by the baby’s metabolism diffuses across the barrier into the mother’s blood, where her lungs exhale it. Urea, the main waste product from protein metabolism, also crosses by passive diffusion. The mother’s kidneys and liver then process and eliminate these waste products alongside her own.
Delivery of the Placenta
After the baby is born, the placenta still needs to be delivered. This is called the third stage of labor. The uterus continues to contract, which causes the placenta to separate from the uterine wall and pass through the birth canal. This stage typically takes about 5 to 6 minutes on average, though it can range from 1 minute to as long as 60 minutes.
Placental Complications
Several conditions can affect how the placenta functions or where it attaches, and they’re among the most closely monitored issues in prenatal care.
Placenta previa occurs when the placenta partially or completely covers the opening of the cervix. It’s typically caught on routine ultrasound and often causes painless vaginal bleeding in the second or third trimester. Many cases diagnosed early in pregnancy resolve on their own as the uterus grows and the placenta shifts upward, but those that persist usually require a cesarean delivery.
Placental abruption is when the placenta separates from the uterine wall before birth. This typically presents with severe abdominal pain and vaginal bleeding, and it’s a medical emergency because it can cut off the baby’s oxygen supply.
Placenta accreta spectrum refers to a group of conditions where the placenta attaches too deeply into the uterine wall. In its mildest form, it simply grows past the normal boundary. In more severe cases, it can invade into the muscle of the uterus or even through it into surrounding organs. This spectrum of conditions makes the placenta difficult or impossible to deliver normally after birth and often requires surgical management. Prior cesarean deliveries are a major risk factor.
Vasa previa is a rarer condition where fetal blood vessels run across or near the cervical opening, unprotected by the placenta or umbilical cord. It poses a serious risk of hemorrhage if those vessels tear during labor.