Fetal Membranes

The fetal membranes are extraembryonic structures that surround, protect, and support the developing embryo and fetus. They participate in formation of the fetal environment, contribute to placental development, facilitate exchange between maternal and fetal compartments, and establish several important temporary structures required during early development.

The principal fetal membranes are the amnion and chorion. In embryological descriptions, the yolk sac and allantois are also considered extraembryonic membranes. These structures arise from tissues associated with the early conceptus but generally do not become incorporated into the body of the embryo itself, except for specific portions or derivatives.

Components of the Fetal Membranes

The major extraembryonic membranes and associated structures include:

• Amnion
• Chorion
• Yolk sac
• Allantois

Each has a distinct developmental origin and function.

Overview of the Fetal Membranes

Structure	Major Relationship	Major Function or Significance
Amnion	Surrounds the amniotic cavity	Creates the fluid-filled environment surrounding the fetus
Chorion	Outermost fetal membrane	Forms chorionic villi and contributes to fetal part of placenta
Yolk sac	Connected to early embryonic gut	Early nutrient transfer, hematopoiesis and germ-cell relationships
Allantois	Extends into connecting stalk	Associated with early umbilical vessels and urachal development

Developmental Origin

The fetal membranes begin developing during the earliest stages following implantation.

They arise from combinations of tissues derived from the embryoblast, trophoblast, and extraembryonic mesoderm.

Relationship to the Conceptus

The term conceptus includes the developing embryo or fetus together with its associated extraembryonic membranes.

The fetal membranes therefore form part of the conceptus even though most of their tissues do not become part of the definitive fetal body.

Amnion

The amnion is the thin innermost fetal membrane surrounding the amniotic cavity.

It develops early in the second week and eventually forms the membrane directly contacting the amniotic fluid surrounding the embryo and fetus.

Formation of the Amnion

A small cavity appears adjacent to the epiblast during early implantation.

Amnioblasts line the side of this cavity facing the trophoblast, while epiblast cells initially form its floor. Together, these relationships establish the early amniotic cavity and amnion.

Amniotic Cavity

The amniotic cavity is the fluid-filled space enclosed by the amnion.

Initially small, it enlarges rapidly and eventually surrounds the entire developing fetus.

Expansion of the Amnion

As the embryo grows and undergoes folding, the amnion expands around it.

The amniotic cavity progressively becomes much larger than the early chorionic cavity and ultimately occupies nearly all of the space surrounding the fetus.

Functions of the Amnion

The amnion contributes to a protected environment in which the fetus develops. Together with amniotic fluid, it:

• Surrounds the embryo and fetus
• Provides space for fetal movement
• Helps protect against mechanical forces
• Reduces abnormal adherence of fetal tissues
• Supports normal musculoskeletal development
• Contributes to maintenance of the intrauterine environment

Amniotic Fluid

Amniotic fluid fills the amniotic cavity.

Its sources and regulation change during pregnancy. Later in gestation, fetal urine is a major contributor, while fetal swallowing is an important route of fluid removal.

Structure of the Amnion

The mature amnion is a thin but mechanically strong and avascular membrane.

It consists of an epithelial layer supported by a basement membrane and connective tissue layers.

Chorion

The chorion is the outermost fetal membrane.

It surrounds the developing embryo, amnion, yolk sac, and early extraembryonic spaces and contributes extensively to formation of the fetal component of the placenta.

Formation of the Chorion

The chorion is formed by the trophoblast together with the underlying extraembryonic somatic mesoderm.

The trophoblast differentiates into cytotrophoblast and syncytiotrophoblast during implantation.

Cytotrophoblast

The cytotrophoblast is the cellular inner component of the trophoblast during early development.

Its proliferating cells contribute to trophoblastic growth and development of chorionic villi.

Syncytiotrophoblast

The syncytiotrophoblast is the multinucleated outer trophoblastic layer that invades the maternal endometrium during implantation.

It contributes to establishment of maternal-fetal relationships and early placental development.

Extraembryonic Somatic Mesoderm

Extraembryonic somatic mesoderm lines the internal surface of the trophoblast and contributes to the structural organization of the chorion.

Together with the trophoblastic layers, it forms the chorionic membrane.

Chorionic Cavity

The chorionic cavity, or extraembryonic coelom, forms within the extraembryonic mesoderm.

During early development, it is a large space surrounding the amnion and yolk sac except where the embryo is attached to the chorion by the connecting stalk.

Connecting Stalk

The connecting stalk is a bridge of extraembryonic mesoderm connecting the early embryo and associated structures to the chorion.

It later contributes to formation of the umbilical cord.

Chorionic Villi

Chorionic villi are projections from the chorion that develop into specialized structures for maternal-fetal exchange.

They undergo primary, secondary, and tertiary stages of development.

Primary Chorionic Villi

Primary chorionic villi consist of cytotrophoblastic cores covered by syncytiotrophoblast.

They represent the earliest structural stage of villous development.

Secondary Chorionic Villi

Secondary chorionic villi form when extraembryonic mesoderm grows into the cores of primary villi.

This creates a mesodermal core surrounded by trophoblastic layers.

Tertiary Chorionic Villi

Tertiary chorionic villi develop when fetal blood vessels form within the mesodermal cores of secondary villi.

These vessels become connected with the developing fetal circulation, allowing the villi to participate in placental exchange.

Chorion Frondosum

The chorion frondosum is the region of the chorion where villi persist and proliferate extensively.

It forms the fetal component of the placenta in association with the maternal decidua basalis.

Chorion Laeve

The chorion laeve, or smooth chorion, develops where chorionic villi regress.

This portion of the chorion becomes relatively smooth and contributes to the nonplacental fetal membranes.

Chorion Frondosum versus Chorion Laeve

Feature	Chorion Frondosum	Chorion Laeve
Villi	Persist and proliferate	Regress
Surface	Villous	Relatively smooth
Major relationship	Placenta	Nonplacental fetal membranes
Maternal association	Decidua basalis	Primarily decidua capsularis and later parietalis

Amnion and Chorion

During early pregnancy, the amnion and chorion are separated over much of their extent by the chorionic cavity.

As the amniotic cavity expands, the amnion approaches the chorion and the intervening space progressively disappears.

Fusion of the Amnion and Chorion

The expanding amnion eventually becomes closely apposed to the chorion, and the membranes fuse to form the amniochorionic membrane.

This fused membrane forms the fetal sac surrounding the amniotic fluid and fetus during later pregnancy.

Amniochorionic Membrane

The amniochorionic membrane consists of the closely associated amnion and chorion.

It is the membrane complex that ruptures when the fetal membranes break during or before labor.

Amnion versus Chorion

Feature	Amnion	Chorion
Position	Innermost fetal membrane	Outermost fetal membrane
Directly surrounds amniotic cavity	Yes	No
Chorionic villi	Absent	Present during development
Placental contribution	Covers fetal placental surface	Chorion frondosum forms fetal placental component
Vascularity	Avascular	Associated with fetal vessels in villi

Yolk Sac

The yolk sac is an important extraembryonic membrane during early embryonic development.

Although humans do not use it as a large nutritional yolk reservoir, it has several essential transient developmental functions.

Primary Yolk Sac

During the second week, hypoblast-derived cells contribute to formation of the exocoelomic membrane, which lines the early cavity and establishes the primary yolk sac.

This structure is subsequently modified as extraembryonic development continues.

Secondary Yolk Sac

The primary yolk sac is replaced by a smaller secondary or definitive yolk sac.

The secondary yolk sac remains associated with the embryo during early development and performs several important temporary functions.

Functions of the Yolk Sac

The yolk sac is associated with:

• Early transfer of nutrients before placental circulation is fully established
• Early blood-cell formation
• Development of early blood vessels
• Origin and migration of primordial germ cells
• Contribution to the primitive gut during embryonic folding

Early Hematopoiesis

The yolk sac is an important site of early embryonic hematopoiesis.

Blood islands develop within its associated mesoderm and contribute to early blood-cell and vascular development.

Primordial Germ Cells

Primordial germ cells become associated with the wall of the yolk sac near the allantois during early development before migrating toward the developing gonads.

These cells ultimately contribute to the germ-cell lineage that produces spermatozoa or oocytes.

Yolk Stalk

As embryonic folding progresses, the connection between the midgut and yolk sac becomes narrowed into the vitelline duct, also called the yolk stalk or omphalomesenteric duct.

This connection normally becomes progressively reduced and disappears.

Vitelline Duct

The vitelline duct temporarily connects the developing midgut to the yolk sac.

Failure of complete obliteration can produce several congenital abnormalities.

Meckel Diverticulum

A Meckel diverticulum can result from persistence of the proximal portion of the vitelline duct.

It arises from the ileum and represents one of the clinically important remnants associated with the embryonic yolk-sac connection.

Allantois

The allantois is a small diverticulum that develops from the caudal region of the yolk sac and extends into the connecting stalk.

In humans it remains relatively small but has important developmental relationships with the umbilical vessels and urinary bladder.

Development of the Allantois

The allantois appears during the third week as an endodermal diverticulum extending into the connecting stalk.

Its surrounding mesoderm becomes associated with developing vessels that contribute to the umbilical circulation.

Allantois and Umbilical Vessels

The developing allantoic region is closely associated with formation of the vessels within the connecting stalk.

These vascular structures contribute to the umbilical vessels linking the fetus with the placenta.

Allantois and the Urinary Bladder

The intraembryonic portion of the allantoic connection becomes associated with the apex of the developing urinary bladder.

It subsequently forms the urachus.

Urachus

The urachus is the fibrous remnant of the channel extending between the fetal urinary bladder and umbilical region.

After birth it normally becomes the median umbilical ligament.

Median Umbilical Ligament

The median umbilical ligament is the postnatal fibrous remnant of the urachus.

It extends from the apex of the urinary bladder toward the umbilicus along the internal surface of the anterior abdominal wall.

Urachal Abnormalities

Incomplete obliteration of the urachus can result in congenital abnormalities including:

• Patent urachus
• Urachal cyst
• Urachal sinus
• Vesicourachal diverticulum

Fetal Membranes and the Placenta

The chorion has a direct structural role in placental formation.

The villous chorion, or chorion frondosum, combines with the maternal decidua basalis to form the placenta.

Fetal Component of the Placenta

The chorion frondosum forms the principal fetal component of the placenta.

Its chorionic villi contain fetal vessels that participate in exchange with maternal blood across the placental barrier.

Maternal Component of the Placenta

The decidua basalis forms the maternal component of the placenta.

It is the region of pregnancy-modified endometrium underlying the implanted conceptus.

Placental Exchange

Maternal and fetal blood normally remain within separate vascular systems.

Exchange of gases, nutrients, waste products, and other substances occurs across tissues separating maternal blood in the intervillous spaces from fetal blood within chorionic villous vessels.

Fetal Membranes and the Umbilical Cord

The expansion of the amnion alters the arrangement of the connecting stalk, yolk-sac connection, and associated vessels.

These structures become incorporated into the developing umbilical cord, whose external surface is covered by amnion.

Umbilical Cord

The umbilical cord connects the fetus to the placenta.

It normally contains two umbilical arteries and one umbilical vein embedded within Wharton jelly and covered externally by amnion.

Wharton Jelly

Wharton jelly is a specialized mucous connective tissue surrounding the umbilical vessels.

It helps protect the vessels from compression and mechanical distortion.

Relationship to the Decidua

The chorion is closely related to the decidua, the pregnancy-modified endometrium.

Different regions of decidua are named according to their relationship to the implanted conceptus.

Decidua Basalis

The decidua basalis lies between the conceptus and the myometrium.

It participates in formation of the maternal component of the placenta.

Decidua Capsularis

The decidua capsularis covers the conceptus on the side facing the uterine cavity during early pregnancy.

As the conceptus enlarges, this region becomes stretched and eventually comes into contact with the decidua parietalis.

Decidua Parietalis

The decidua parietalis lines the remaining uterine wall away from the implantation site.

Expansion of the gestational sac eventually brings the decidua capsularis into contact with it, contributing to obliteration of the uterine cavity as a free space during pregnancy.

Fetal Membranes and the Uterine Cavity

As pregnancy progresses, the expanding conceptus occupies an increasing proportion of the uterine cavity.

The chorion laeve and associated membranes eventually become closely related to the decidual lining of the uterus.

Membrane Rupture

The fused amnion and chorion normally remain intact through most of pregnancy.

Rupture of membranes occurs when these membranes break, allowing amniotic fluid to escape.

Rupture During Labor

Rupture of the fetal membranes commonly occurs shortly before or during labor.

The timing relative to labor and gestational age has important clinical significance.

Prelabor Rupture of Membranes

Prelabor rupture of membranes occurs when the membranes rupture before labor begins.

If it occurs before 37 completed weeks of gestation, it is termed preterm prelabor rupture of membranes.

Effects of Membrane Rupture

Membrane rupture can result in loss of amniotic fluid and removal of part of the physical barrier separating the intrauterine environment from the lower reproductive tract.

The clinical consequences depend strongly on gestational age, duration of rupture, fetal status, and the presence of infection.

Ascending Infection

Intact fetal membranes contribute to protection of the intrauterine environment from ascending microorganisms.

Once the membranes rupture, ascending infection becomes an important clinical concern, particularly when the interval between rupture and delivery is prolonged.

Chorioamnionitis

Chorioamnionitis refers to inflammation, often associated with infection, involving the chorion and amnion.

Intraamniotic infection can involve the fetal membranes, amniotic fluid, placenta, fetus, or combinations of these structures.

Amnionitis

Inflammation can involve the amnion as part of an ascending inflammatory process affecting the fetal membranes.

Histological examination of membranes and placenta can demonstrate inflammatory changes when infection has occurred.

Amniotic Band Sequence

Amniotic band sequence involves fibrous strands that can entangle or constrict developing fetal structures.

The resulting abnormalities may include constriction rings, distal edema, limb defects, or more extensive disruptions depending on the tissues involved.

Chorionic Villus Sampling

Chorionic villus sampling obtains tissue derived from chorionic villi for prenatal diagnostic testing.

Because chorionic villi are fetal in origin, their cells can provide information about the fetal genetic complement.

Amniocentesis

Amniocentesis involves sampling amniotic fluid from the amniotic cavity using image-guided needle placement.

Fetal-derived cells and substances within the fluid can be analyzed for selected prenatal diagnostic purposes.

Fetal Membranes in Multiple Pregnancy

The arrangement of the chorion and amnion is particularly important in multiple gestations.

Twins may have separate or shared chorionic and amniotic compartments depending on zygosity and, in monozygotic twins, the timing of embryonic division.

Dichorionic Diamniotic Twins

Dichorionic diamniotic twins have two chorions and two amniotic sacs.

All dizygotic twin pregnancies are dichorionic diamniotic, while some monozygotic pregnancies also have this arrangement when embryonic division occurs early.

Monochorionic Diamniotic Twins

Monochorionic diamniotic twins share a chorion but have separate amniotic sacs.

This arrangement occurs in monozygotic twinning when embryonic division occurs after chorionic development has begun but before formation of a shared amniotic cavity.

Monochorionic Monoamniotic Twins

Monochorionic monoamniotic twins share both a chorion and an amniotic cavity.

This arrangement is less common and is associated with clinically important risks related to sharing the same amniotic space.

Chorionicity

Chorionicity describes the number and arrangement of chorions in a multiple pregnancy.

Determining chorionicity is clinically important because shared placental structures influence the risks and management of twin pregnancies.

Amnionicity

Amnionicity describes whether multiple fetuses occupy separate or shared amniotic sacs.

It is distinct from chorionicity and provides additional information about fetal membrane organization.

Developmental Sequence

1. The blastocyst implants into the endometrium.
2. The embryoblast differentiates into epiblast and hypoblast.
3. The amniotic cavity develops adjacent to the epiblast.
4. Amnioblasts contribute to formation of the amnion.
5. The early yolk sac develops in association with the hypoblast.
6. Extraembryonic mesoderm develops around the early amnion and yolk sac.
7. Cavities develop within the extraembryonic mesoderm and form the chorionic cavity.
8. The trophoblast and extraembryonic somatic mesoderm establish the chorion.
9. Chorionic villi develop.
10. The connecting stalk maintains attachment between the embryo and chorion.
11. The allantois extends into the connecting stalk.
12. The embryo undergoes folding.
13. Part of the yolk sac is incorporated into the developing gut.
14. The yolk-sac connection narrows to form the vitelline duct.
15. The amnion expands around the embryo and umbilical structures.
16. The amniotic cavity enlarges progressively.
17. The chorionic cavity becomes progressively smaller.
18. The amnion becomes apposed to and fuses with the chorion.
19. The chorion frondosum contributes to the fetal placenta.
20. The chorion laeve contributes to the smooth nonplacental fetal membranes.
21. The fused amnion and chorion surround the fetus and amniotic fluid during later pregnancy.

Key Anatomical and Developmental Relationships

• The amnion is the innermost fetal membrane.
• The chorion is the outermost fetal membrane.
• The amnion directly surrounds the amniotic cavity.
• The chorion develops from trophoblast and extraembryonic somatic mesoderm.
• Chorionic villi contribute to placental development.
• The chorion frondosum forms the fetal component of the placenta.
• The chorion laeve forms the smooth nonplacental chorion.
• The chorionic cavity is prominent during early development but is later obliterated as the amnion expands.
• The amnion and chorion eventually form the amniochorionic membrane.
• The yolk sac has important roles in early hematopoiesis and germ-cell development.
• Part of the yolk sac is incorporated into the primitive gut.
• The vitelline duct temporarily connects the midgut and yolk sac.
• The allantois extends into the connecting stalk.
• The allantoic connection contributes to development of the urachus.
• The urachus later becomes the median umbilical ligament.
• The umbilical cord is externally covered by amnion.
• Rupture of the amnion and chorion allows amniotic fluid to escape.

Clinical Importance

The fetal membranes establish and maintain the extraembryonic environment required for normal embryonic and fetal development. The amnion provides the immediate fluid-filled compartment surrounding the fetus, while the chorion establishes critical relationships with maternal tissues and contributes directly to formation of the placenta.

The yolk sac and allantois are particularly important during early development. Although they become greatly reduced later in pregnancy, they participate in processes including early hematopoiesis, primordial germ-cell development, primitive gut formation, umbilical vascular development, and formation of structures related to the urinary bladder.

Abnormalities involving these membranes can have important clinical consequences. Premature membrane rupture can result in fluid loss and increased risk of ascending infection, persistent embryonic connections can produce vitelline or urachal abnormalities, and the arrangement of the chorion and amnion determines important anatomical categories of multiple pregnancy. Understanding the fetal membranes therefore provides a foundation for studying embryology, amniotic fluid, placental development, umbilical cord formation, implantation, multiple pregnancy, congenital anomalies, prenatal diagnosis, and membrane rupture.