The chorion is the outermost membrane surrounding a developing embryo, and in humans it plays a central role in forming the placenta, protecting the fetus, and mediating the exchange of gases and nutrients between mother and baby. It begins taking shape within the first two weeks after conception and remains a critical structure throughout pregnancy. But the chorion is not unique to humans. Versions of it appear across the animal kingdom, from bird eggs to fish embryos, each adapted to solve a different set of survival problems.
How the Chorion Forms
In human pregnancy, the chorion starts to develop around day 14 after conception, when a network of cells derived from the embryonic disc spreads beneath the inner surface of the implanted blastocyst and creates what is called the chorionic sac. This sac effectively envelops the embryo and is the first major barrier between the developing life and the mother’s uterine wall. By about the eighth week of pregnancy, the chorionic sac divides into two distinct structures. Finger-like projections called villi on the side of the sac facing the uterine wall grow rapidly, branch extensively, and mature into the placenta. Meanwhile, the villi on the opposite side, the side facing the uterine cavity, stop growing and gradually smooth out, forming what is known as the smooth chorion, or chorion laeve.1Elsevier / Placenta. The role of chorionic cytotrophoblasts in the smooth chorion fusion with parietal decidua
This split matters because it means the placenta and the smooth chorion share an origin but take on very different jobs. The placenta becomes the metabolic engine of pregnancy, while the smooth chorion fuses with the amnion (the innermost membrane directly around the fetus) to form part of what people commonly call “the bag of waters.” Together, these membranes hold amniotic fluid in place, cushion the fetus, and serve as a selective barrier against infection.
What the Chorion Actually Does
The chorion’s most talked-about function is its role in building the placenta, but its contributions go beyond that single task. Where the chorion retains its active villi and develops into the placenta, it facilitates the transfer of oxygen and nutrients from the mother’s blood to the fetus and carries carbon dioxide and other waste products back in the other direction.2Thrombosis Research. Growth and function of the normal human placenta This exchange happens across layers of specialized cells called trophoblasts, which line the chorionic villi and sit in direct contact with maternal blood.
Trophoblasts also produce key pregnancy hormones. Human chorionic gonadotropin, or hCG, is the hormone that pregnancy tests detect. It is synthesized in the syncytiotrophoblast layer of the chorionic villi and is found at its highest concentrations in early pregnancy. Another hormone, human placental lactogen (hPL), peaks later and helps regulate the mother’s metabolism to ensure a steady fuel supply to the fetus. Both hormones are made and packaged through the same cellular machinery within the syncytiotrophoblast, with only small amounts stored at any given time before being released into the mother’s bloodstream.3PubMed Central. An immunogold, cryoultrastructural study of sites of synthesis and storage of chorionic gonadotropin and placental lactogen in human syncytiotrophoblast
Immune Protection at the Boundary
Pregnancy presents an immunological puzzle. The fetus carries half its genetic material from the father, which means it is, from the mother’s immune system’s perspective, partly foreign tissue. The chorion and its trophoblast cells are at the front line of managing this tension. One of the ways they do this is by expressing a molecule called HLA-G on the surface of invasive trophoblast cells. HLA-G acts as a kind of molecular passport, signaling to the mother’s natural killer (NK) cells that the trophoblasts should not be attacked. When researchers knocked out HLA-G expression in trophoblast cells in lab experiments, NK cells quickly destroyed them, confirming that HLA-G is a key regulator of this truce between maternal immune defenses and fetal tissue.4PubMed. Inhibition of HLA-G expression via RNAi abolishes resistance of extravillous trophoblast cell line TEV-1 to NK lysis
Without this immune-shielding function, the mother’s body would treat the placenta like an intruder. Failures in immune tolerance at the chorion-decidua interface are thought to contribute to complications like preeclampsia and recurrent miscarriage, though the full picture involves many other immune pathways working alongside HLA-G.
Chorion Sampling in Prenatal Diagnosis
Because chorionic villi contain fetal DNA, they can be sampled for genetic testing early in pregnancy, a procedure called chorionic villus sampling (CVS). During CVS, a small piece of placental tissue is taken, usually between 10 and 13 weeks of gestation, and the fetal chromosomes are analyzed. This allows parents and doctors to detect chromosomal conditions far earlier than amniocentesis, which typically happens around 15 to 20 weeks.
A large prospective study comparing the two procedures at 10 to 13 weeks found that both had the same success rate for obtaining a usable sample, about 97.5%. However, the rate of spontaneous pregnancy loss was lower after CVS than after early amniocentesis performed at the same gestational age. In that study, intrauterine or neonatal death after early amniocentesis occurred in roughly 5% of cases, compared to about 2% after CVS.5PubMed. Comparison of chorionic villus sampling and amniocentesis for fetal karyotyping at 10-13 weeks’ gestation It is worth noting that early amniocentesis (before 14 weeks) is no longer standard practice in most settings, partly because of findings like these. The comparison most parents face today is between first-trimester CVS and second-trimester amniocentesis, where the risk profiles are closer together.
Why Chorionicity Matters in Twin Pregnancies
When twins develop in the womb, one of the most important questions doctors ask is whether the twins share a chorion or each have their own. Twins who share a single chorion are called monochorionic, and this arrangement creates risks that dichorionic twins (each with a separate chorion and placenta) generally do not face.
The core issue is blood vessel connections. In a monochorionic placenta, the two fetal circulations are linked by vascular anastomoses, shared blood vessels that allow blood to flow between the twins. This shared circulation means one twin’s well-being depends directly on the other’s. It also sets the stage for complications unique to monochorionic pregnancies, including twin-to-twin transfusion syndrome (TTTS), where blood flow becomes unbalanced and one twin receives too much while the other receives too little.6PubMed. The vascular anastomoses in monochorionic twin pregnancies and their clinical consequences In one study of monochorionic twins, TTTS occurred in about 29% of cases, and roughly three-quarters of monochorionic babies had low birth weight.7PubMed Central. Perinatal outcome of monochorionic in comparison to dichorionic twin pregnancies
Determining chorionicity is usually done by ultrasound in the first trimester, and it shapes the entire monitoring plan for the pregnancy. Monochorionic twins are typically scanned every two weeks to watch for early signs of TTTS and other complications, while dichorionic twins can often be monitored less intensively.
Chorioamnionitis and Membrane Infection
When bacteria reach the chorion and amnion, the resulting infection is called chorioamnionitis. This is one of the more common complications of premature rupture of the membranes (when the “water breaks” too early), and it poses risks to both mother and baby, including preterm delivery, sepsis, and long-term neurological problems in the infant.
The infection triggers a cascade of inflammation. Research has shown that women with chorioamnionitis after premature membrane rupture have elevated levels of inflammatory markers in their blood, including IL-1β, CD14, and NFKB1. Importantly, these markers increase dynamically over time in affected women, which has sparked interest in using blood-based molecular tests to detect chorioamnionitis earlier and more accurately than traditional clinical signs like fever and elevated heart rate.8PubMed Central. Chorioamnionitis Occurring in Women With Preterm Rupture of the Fetal Membranes Is Associated With a Dynamic Increase in mRNAs Coding Cytokines in the Maternal Circulation The clinical challenge is that traditional signs can be subtle or absent, so by the time chorioamnionitis is diagnosed, the infection may already be well established.
Placenta Accreta and Abnormal Invasion
Normally, a thin layer of tissue called the decidua sits between the chorionic villi and the muscular wall of the uterus. The decidua acts as a controlled landing zone, allowing the placenta to implant firmly enough for nutrient exchange but preventing it from burrowing too deep. In placenta accreta spectrum (PAS), this boundary fails. The chorionic villi attach directly to, or invade into, the muscular layer of the uterus because the decidua is absent or deficient.9PubMed Central. Placenta Accreta Spectrum: A Review of Pathology, Molecular Biology, and Biomarkers
The trophoblast cells in these cases behave differently from normal ones. They retain a more invasive character, failing to undergo the normal shift toward a less aggressive state as pregnancy progresses.10PubMed Central. Trophoblast invasion: Lessons from abnormally invasive placenta (placenta accreta) Prior cesarean sections are the biggest risk factor, because the scar tissue left behind disrupts the normal uterine lining and can promote excessive trophoblast invasion and abnormal blood vessel remodeling.11Human Reproduction. L26/P-916 Modeling scar fibroblast–mediated pathological vascular remodeling and excessive trophoblast invasion using a vascular–trophoblast organoid assembloid system for cesarean scar pregnancy and placenta accreta spectrum PAS is dangerous because the placenta cannot detach cleanly after delivery, leading to severe bleeding that often requires surgery and, in severe cases, hysterectomy.
Chorion-Based Grafts for Wound Healing
Outside of pregnancy, the chorion and amnion have found a second life in medicine as biological wound dressings. Processed membranes from donated placentas are used to treat chronic, hard-to-heal wounds like diabetic ulcers and venous leg ulcers. The membranes contain growth factors and anti-inflammatory molecules that promote tissue repair, reduce scarring, and attract stem-like progenitor cells to the wound site.12PubMed Central. Biological properties of dehydrated human amnion/chorion composite graft: implications for chronic wound healing
A study of 32 patients with 53 chronic non-healing wounds treated with dehydrated amnion/chorion membrane found that about 72% of wounds showed a favorable outcome, defined as 70% or greater reduction in wound area. Roughly two-thirds of the wounds healed completely, with a median healing time of 77 days.13PubMed Central. Dehydrated human amnion/chorion membrane allograft with spongy layer to significantly improve the outcome of chronic non-healing wounds These results are encouraging for wounds that have resisted other treatments for months or years.
Interestingly, amnion and chorion membranes do not perform identically. A comparative study of facial wound healing found that amnion membrane alone produced faster results, with complete healing by the second week, while chorion membrane left wounds still unhealed at that point.14PubMed Central. Efficacy of Amniotic and Chorionic Membrane in Facial Wound Healing: A Comparative Study The two membranes have different compositions, with the amnion being thinner and richer in certain growth factors, while the chorion provides more structural scaffolding. In practice, many commercial products use a composite of both layers to combine the advantages of each.
The Chorion Across the Animal Kingdom
The chorion is not a human invention. It is one of four fetal membranes, along with the amnion, allantois, and yolk sac, that evolved in the ancestors of all amniote vertebrates (reptiles, birds, and mammals). The development of these membranes was what allowed vertebrates to reproduce on land, freeing them from the need to lay eggs in water.15PubMed. Evolution and development of fetal membranes and placentation in amniote vertebrates From this shared starting kit, different animal lineages have put the chorion to very different uses.
In birds, the chorion fuses with the allantois to form the chorioallantoic membrane (CAM), which lines the inside of the eggshell. The CAM handles respiratory gas exchange, pulling oxygen in and pushing carbon dioxide out through tiny pores in the shell. It also dissolves and transports calcium from the eggshell into the developing chick’s bones, manages the embryo’s acid-base balance, and reabsorbs water and ions from the fluid surrounding the embryo.16PubMed Central. The chick chorioallantoic membrane: a model of molecular, structural, and functional adaptation to transepithelial ion transport and barrier function during embryonic development Research on chick embryos has identified specific genes that coordinate this calcium transport, switching on around day 13 of incubation when the skeleton’s calcium demands ramp up.17Food Science and Human Wellness. Transcriptome-based insights into the calcium transport mechanism of chick chorioallantoic membrane
In fish, the word “chorion” refers to the tough outer shell of the egg rather than a fetal membrane in the mammalian sense. The fish chorion forms an elastic protective barrier around the embryo, shielding it from physical damage, drying out, and sudden environmental shifts. It also plays roles in nutrient uptake, fertilization (sperm must penetrate it through a specialized pore), and defense against pathogens until the embryo hatches.18ScienceDirect (Aquaculture Reports). Chorion in fish: Synthesis, functions and factors associated with its malformations Even in insects, a chorionic layer exists. In the eggs of certain beetles, for example, the chorion has an outer surface dotted with respiratory openings called aeropyles, which allow the embryo to breathe while remaining sealed inside the egg.19PubMed Central. Morphology of the egg shell and the developing embryo of the Red Palm Weevil, Rhynchophorus ferrugineus
The common thread is that the chorion, in all its forms, sits at the boundary between an embryo and the outside world and manages what crosses that boundary. Whether it is ferrying oxygen and calcium through a chicken eggshell, negotiating immune tolerance with a human mother, or shielding a fish embryo from waterborne pathogens, the chorion’s job is always to keep the embryo alive in its particular environment. The details differ wildly, but the underlying logic is the same.
Chorion-Related Research in Regenerative Medicine
Beyond wound dressings, chorion-derived cells and tissues are being explored for broader applications in regenerative medicine. The chorionic membrane contains mesenchymal stromal cells with properties similar to stem cells, making them attractive candidates for tissue engineering and cell therapy. Because the chorion is collected from placentas after delivery, the material is abundant and poses fewer ethical concerns than other stem cell sources.
One active area involves differentiating trophoblasts and villous tissue from human embryonic stem cells or induced pluripotent stem cells in the lab. Researchers have successfully generated normally differentiated villous structures using these methods, creating models that can be used for research into conditions like fetal growth restriction and pregnancy-related high blood pressure without requiring invasive procedures on pregnant women.20PubMed Central. Human Chorionic Villous Differentiation and Placental Development These lab-grown models are still in early stages, but they open the door to studying placental diseases and testing potential treatments in ways that were previously impossible. The chorion, it turns out, keeps finding new jobs even after pregnancy ends.