Gestational sac
Encyclopedia
The gestational sac is the only available intrauterine
structure that can be used to determine if an intrauterine pregnancy
(IUP) exists, until the embryo
is identified.
On ultrasound
, it is an anechoic (dark) space surrounded by a hyperchoic (white) rim.
It is spherical
in shape, and usually located in the upper uterine fundus.
The mean gestational sac diameter (MGD) is an effective estimate of gestational age between 5 and 6 weeks, with an accuracy of about +/- 5 days.
The yolk sac
and embryo
should be readily identified when the gestational sac reaches a certain size -- a yolk sac should be seen when gestational sac is 20mm and a fetal pole should be seen when the gestational sac reaches 25mm.
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
structure that can be used to determine if an intrauterine pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
(IUP) exists, until the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
is identified.
On ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
, it is an anechoic (dark) space surrounded by a hyperchoic (white) rim.
It is spherical
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
in shape, and usually located in the upper uterine fundus.
The mean gestational sac diameter (MGD) is an effective estimate of gestational age between 5 and 6 weeks, with an accuracy of about +/- 5 days.
The yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
and embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
should be readily identified when the gestational sac reaches a certain size -- a yolk sac should be seen when gestational sac is 20mm and a fetal pole should be seen when the gestational sac reaches 25mm.
Characteristics on ultrasound
Gestational sacs can be identified via ultrasound and are generally identified by the following 4 characteristics:- Round or elliptical shape in longitudinal and transverse views
- Surrounded by an echogenic rim (choriodecidual reaction)
- Gestational sac is in uterine fundus
- Sac is not directly midline, but implanted eccentrically (to one side of the uterine cavity line, without displacing it).