What does chorion mean?
The chorion is the outermost of the membranes that enclose an embryo during development. In humans and other mammals, it is a double-layered structure — an outer layer derived from the trophoblast and an inner somatic mesoderm — and it plays a central role in forming the fetal portion of the placenta, through which oxygen, nutrients, and waste pass between mother and child. Its finger-like projections, the chorionic villi, dramatically increase the surface area available for this exchange and are the tissue sampled in prenatal tests such as chorionic villus sampling. In birds and reptiles, the chorion lies against the shell and assists in respiration. The adjective form is 'chorionic'. The word appears almost exclusively in medical, biological, and veterinary contexts, making it unfamiliar to general readers but essential vocabulary in reproductive science.
nounThe outermost of the two membranes surrounding an embryo in the uterus, which in mammals contributes to the formation of the placenta.
- The outermost membrane enclosing the embryo in reptiles, birds, and mammals, forming the fetal part of the placenta in mammals.
"The ultrasound revealed that the chorion had developed normally around the embryo."
"During the first weeks of pregnancy, the chorion surrounds the developing embryo and begins forming the placental tissue."
"In birds and reptiles, the chorion lies just beneath the eggshell and plays a role in gas exchange for the growing embryo."
Rarely used in the plural, since each pregnancy involves a single chorion; the plural may appear when discussing multiple embryos or comparative anatomy across species.
"In twin pregnancies with separate placentas, each embryo develops within its own chorion."
The chorion is the very first placenta-in-waiting — before your heart even finished forming, this membrane was already negotiating nutrients between you and your mother.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chorion
Chorion comes from the Greek 'khorion', meaning the outer membrane or afterbirth, and entered English via New Latin during the early modern period as anatomical terminology was being standardized. The adjective 'chorionic' derives directly from it and is far more common in clinical usage today, appearing in established terms such as 'human chorionic gonadotropin' (hCG), the hormone detected by pregnancy tests.
Related word forms
How chorion is actually used
Strictly a technical term used in embryology, obstetrics, and veterinary science; it rarely appears outside medical or scientific writing. It is almost always used in its singular form.
Easily confused with chorion
The amnion is the inner membrane closest to the fetus, while the chorion is the outermost membrane lying against the uterine wall.
Corium refers to the dermis, the layer of skin beneath the epidermis, whereas chorion refers to an embryonic membrane.