[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Fetal period, placenta and twins](https://durgical.com/en/embryology/#a-fetal) › Fetal membranes: amnion, chorion, yolk sac and allantois [Embryology](https://durgical.com/en/embryology/) · [Fetal period, placenta and twins](https://durgical.com/en/embryology/#a-fetal) # Fetal membranes: amnion, chorion, yolk sac and allantois - Embryonic structure Four structures form around the embryo: the amnion with its fluid, the chorion that builds the placenta, the yolk sac and the allantois. Updated 2026-11-01 **See it in 3D:** [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) ## Key points - Amnion: derived from epiblast; wraps the embryo and cord and holds the amniotic fluid. - Chorion: trophoblast plus extraembryonic mesoderm; its bushy part forms the placenta and its smooth part (chorion laeve) stays as membrane. - By month 3 the amnion and chorion fuse and the chorionic cavity disappears. - Yolk sac: source of primordial germ cells and the first blood formation (week 3). - Allantois: its vessels become the umbilical vessels; its lumen gives the urachus, then the median umbilical ligament. ## What it is The fetal membranes come from the zygote but are not part of the embryo's body. The amnion makes the fluid-filled cavity, the chorion gives rise to the placenta, the yolk sac supplies germ cells and the first blood, and the allantois leaves the umbilical vessels and the urachus. Knowing them explains twin chorionicity and several embryonic remnants in the adult. ## Structure - Week 2: the amniotic cavity appears within the epiblast and the chorionic cavity in the extraembryonic mesoderm. - The chorion frondosum, at the embryonic pole, keeps its villi; the chorion laeve, at the opposite pole, loses them. - The decidua is divided into basalis (under the embryo), capsularis (over it) and parietalis (the rest of the uterus). - The expanding amnion pushes the yolk sac and allantois into the umbilical cord. ## What it becomes - Amnion and chorion laeve: form the «bag of waters» that ruptures in labour. - Chorion frondosum: the fetal part of the placenta. - Yolk sac: gives the endodermal roof of the primitive gut; its duct closes off (remnant: Meckel diverticulum). - Allantois: urachus and median umbilical ligament; its vessels, the umbilical arteries and vein. ## Clinical notes - Amniotic bands: torn amnion can wrap fingers or limbs and cause amputations or constrictions. - Premature rupture of membranes: risk of preterm birth and infection. - On early ultrasound, the number of chorionic and amniotic sacs defines twin chorionicity. - Patent urachus: urachal fistula, cyst or sinus in the lower midline. ## Memory aid Amnion protects, chorion feeds, yolk sac seeds and allantois leaves the vessels: one job per membrane. ## Test yourself Think of the answer, then tap the question to see it. ## Which part of the chorion forms the placenta? The chorion frondosum, at the embryonic pole, joined to the decidua basalis. ## Which two early roles does the yolk sac play? It is where primordial germ cells arise and where blood formation first starts, from week 3. ## What does the allantois become? Its lumen gives the urachus, later the median umbilical ligament, and its vessels form the umbilical vessels. ## When do the amnion and chorion fuse? Around the end of month 3, when the growing amnion obliterates the chorionic cavity. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/fetal-membranes-amnion-chorion-yolk-sac-and-allantois/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) ## Related - [Week 2: the bilaminar disc ('the week of twos')](https://durgical.com/en/embryology/week-2-the-bilaminar-disc/) - [Placenta: structure, circulation and functions](https://durgical.com/en/embryology/placenta-structure-circulation-and-functions/) - [Umbilical cord: two arteries, one vein](https://durgical.com/en/embryology/umbilical-cord-two-arteries-one-vein/) - [Amniotic fluid: polyhydramnios and oligohydramnios](https://durgical.com/en/embryology/amniotic-fluid-polyhydramnios-and-oligohydramnios/) - [Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusion](https://durgical.com/en/embryology/twins-dizygotic-and-monozygotic-chorionicity-twin-twin-transfusion/) - [Meckel diverticulum and other vitelline duct remnants](https://durgical.com/en/embryology/meckel-diverticulum-and-other-vitelline-duct-remnants/) - [Urachal anomalies: patent urachus, cyst and sinus](https://durgical.com/en/embryology/urachal-anomalies-patent-urachus-cyst-and-sinus/) - [Gametogenesis: spermatogenesis and oogenesis](https://durgical.com/en/embryology/gametogenesis-spermatogenesis-and-oogenesis/) ## Related conditions - [Premature rupture of membranes (PROM and PPROM)](https://durgical.com/en/conditions/premature-rupture-of-membranes/) [← All of embryology](https://durgical.com/en/embryology/) Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given. Sources: [Amniotic fluid: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002220.htm) (MedlinePlus, U.S. National Library of Medicine, 2024); Langman's Medical Embryology, 13th ed., ch. 8: Third Month to Birth: The Fetus and Placenta (Wolters Kluwer, 2015). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/fetal-membranes-amnion-chorion-yolk-sac-and-allantois/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/fetal-membranes-amnion-chorion-yolk-sac-and-allantois/