The gestational sac is the large cavity of

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

surrounding the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

. During early

embryogenesis An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The uter ...

. It is the only available structure that can be used to determine if an intrauterine

pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ca ...

exists until the

can be identified. On

obstetric ultrasound Obstetric ultrasonography, or prenatal ultrasound, is the use of medical ultrasonography in pregnancy, in which sound waves are used to create real-time visual images of the developing embryo or fetus in the uterus (womb). The procedure is a ...

, the gestational sac is a dark (

anechoic An anechoic chamber (''an-echoic'' meaning "non-reflective") is a room designed to stop reflections of either sound or electromagnetic waves. They are also often isolated from energy entering from their surroundings. This combination means t ...

) space surrounded by a white (

hyperechoic Echogenicity (misspelled sometimes as echogenecity) or echogeneity is the ability to bounce an echo, e.g. return the signal in ultrasound examinations. In other words, echogenicity is higher when the surface bouncing the sound echo reflects increa ...

) rim.

Structure

The gestational sac is

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

in shape, and is usually located in the upper part (fundus) of the

. By approximately nine weeks of

gestational age In obstetrics, gestational age is a measure of the age of a pregnancy which is taken from the beginning of the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available. Su ...

, due to folding of the

trilaminar germ disc A trilaminar embryonic disc (or trilaminary blastoderm, or trilaminar germ disk) is an early stage in the development of triploblastic organisms, which include humans and many other animals. It is the next stage from the earlier bilaminar embryo ...

, the

amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shor ...

expands and occupy the majority of the volume of the gestational sac, eventually reducing the extraembryonic coelom (the gestational sac or the chorionic cavity) to a thin layer between the parietal somatopleuric and visceral splanchnopleuric layer of extraembryonic mesoderm.

Development

During embryogenesis, the ''extraembryonic coelom'' (or ''chorionic cavity'') that constitutes the gestational sac is a portion of the

conceptus A conceptus (from Latin: ''concipere'' to conceive) is an embryo and its appendages (adnexa), the associated membranes, placenta, and umbilical cord; the products of conception or, more broadly, "the product of conception at any point between fert ...

consisting of a cavity between

Heuser's membrane Heuser's membrane (or the exocoelomic membrane) is a short lived combination of hypoblast cells and extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of ...

and the

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

. During formation of the primary yolk sac, some of the migrating

hypoblast In amniote embryology, the hypoblast, is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to ...

cells differentiate into mesenchymal cells that fill the space between

and the

, forming the extraembryonic

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...

. As development progresses, small lacunae begin to form within the extraembryonic

which enlarges to become the extraembryonic coelom. The extraembryonic coelom divides the extraembryonic

into two layers: extraembryonic splanchnopleuric

, which lies adjacent to Heuser's membrane around the outside of the primary yolk sac, and extraembryonic somatopleuric

, which lies adjacent to the cytotrophoblast layer of the embryo. The chorionic cavity is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells.

Clinical significance

Ultrasound

Gestational sac diameter by gestational age

The mean sac diameter can effectively estimate the

between 5 and 6 weeks, with an accuracy of about +/- 5 days. The

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far ...

and

should be readily identifiable when the gestational sac reaches a certain size — a yolk sac should be seen when the gestational sac is 20mm and a

fetal pole The fetal pole is a thickening on the margin of the yolk sac of a fetus during pregnancy. It is usually identified at six weeks with vaginal ultrasound and at six and a half weeks with abdominal ultrasound. However, it is not unheard of for the f ...

should be seen when the gestational sac reaches 25mm. Gestational sacs can be identified via ultrasound and are generally identified by the following four characteristics: # The sac has a round or elliptical shape in longitudinal and transverse views # The sac is surrounded by a white

echogenic Echogenicity (misspelled sometimes as echogenecity) or echogeneity is the ability to bounce an echo, e.g. return the signal in ultrasound examinations. In other words, echogenicity is higher when the surface bouncing the sound echo reflects increa ...

rim (''choriodecidual reaction'') # The sac is located in the uterine fundus # The sac is not implanted on the midline, but eccentrically (to one side of the uterine cavity line).

References

External links