gestational sac

(noun)

The gestational sac (or gestation sac) is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy (IUP) exists until the embryo is identified.

Related Terms

Examples of gestational sac in the following topics:

  • Yolk Sac Development

    • The yolk sac is the first element seen in the gestational sac during pregnancy.
    • In humans, it is usually visible at five weeks of gestation.
    • Identifying a true gestation sac is a critical landmark, and is reliably seen in early pregnancy through ultrasound.
    • The yolk sac starts forming during the second week of embryonic development, at the same time of the shaping of the amniotic sac.
    • The yolk sac is a membranous sac attached to the embryo that provides nourishment in the form of yolk.

  • Development of the Extraembryonic Coelom

    • During the formation of the primitive yolk sac, some of the migrating hypoblast cells transdifferentiate into mesenchymal cells that fill the space between Heuser's membrane and the trophoblast to form the extra-embryonic mesoderm.
    • The extra-embryonic mesoderm is divided into two layers: the extra-embryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primitive yolk sac; and the extra-embryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.
    • An artificially colored image of the contents in the cavity of the uterus seen at approximately 5 weeks of gestational age by obstetric ultrasonography.

  • Early Pregnancy Tests

    • With obstetric ultrasonography the gestational sac sometimes can be visualized as early as four and a half weeks of gestation (approximately two and a half weeks after ovulation) and the yolk sac at about five weeks' gestation.
    • The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks' gestation.

  • Fifth Through Eighth Weeks of Development

    • Weeks five to eight of gestation develops the major organs, including the circulatory, nervous, and gastrointestinal systems.
    • Gestational age is the time that has passed since the onset of the last menstruation, which occurs two weeks before the actual fertilization.
    • Thus, the first week of embryonic age is already week three counting with gestational age.
    • Rudimentary blood begins to move through the main embryonic blood vessels, connecting to the yolk sac and the chorionic membrane of the placenta.
    • By week eight of gestation, the embryo measures 13 millimeters in length.

  • Fourth Week of Development

    • The fourth week of gestation is characterized by the flexion of the superior portion of the neural tube to create the mesencephalon.
    • At the end of the fourth week the yolk sac presents the appearance of a small pear-shaped vesicle (the umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct.
    • Late in the fourth week of gestation, the superior part of the neural tube flexes at the level of the future midbrain, the mesencephalon.

  • Fetal Development

    • At the end of the 10th week of gestation, the fetal period begins.
    • In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the ninth week after fertilization.
    • Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and a list of changes by weeks of gestational age.
    • The lungs are developing branches of the respiratory tree as well as cells that produce surfactant, a substance that will help the air sacs inflate once born.

  • Characteristics of Amniotes

    • Instead, the embryo grows within the mother's body; however, even with this internal gestation, amniotic membranes are still present.
    • While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac.

  • Lymphoid Tissue

    • Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm, the inner tissue layer of the embryo.
    • The lymphatic endothelial cells proliferate into sacs that eventually become lymph nodes, with afferent and efferent vessels that flow out from the lymph nodes.
    • More specialized primary lymph tissue, such as the thymus, develops from pharyngeal pouches (embryonic structures that differentiate into organs near the pharynx and throat) by the eighth week of gestation.

  • Metabolic Changes

    • Maternal insulin resistance can lead to gestational diabetes.
    • Gestational diabetes (or gestational diabetes mellitus, GDM) is a condition in which women without previously diagnosed diabetes exhibit high blood glucose levels during pregnancy (especially during the third trimester).
    • Gestational diabetes is caused when the insulin receptors do not function properly.
    • Gestational diabetes generally has few symptoms and it is most commonly diagnosed by screening during pregnancy.
    • Babies born to mothers with untreated gestational diabetes are typically at increased risk of problems such as being large for gestational age (which may lead to delivery complications), low blood sugar, and jaundice.

  • Lymphatic Tissue Development

    • The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins.
    • The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine.
    • The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins.
    • The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.
    • With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes .