Yolk sac
Updates to Article Attributes
The yolk sac is the first anatomical structure identified within the gestational sac. It plays a critical role in embryonal development by providing nutrients, serving as the site of initial haematopoiesis, and contributing to the development of fetal gastrointestinal and reproductive systems 2.
Radiographic features
Ultrasound
- yolk sac appears as a circular transonic mass within gestational sac
- yolk sac should be seen when the mean sac diameter (MSD) is 20 mm on transabdominal scanning and is usually seen endovaginally with an MSD of 8-10 mm.
- visualization of a yolk sac is useful in distinguishing an intrauterine pregnancy (IUP) from a pseudogestational sac, a decidual cast cyst or an anembryonic pregnancy, as a yolk sac is only seen in an intrauterine pregnancy.
In general if the MSD is ≥ 25≥25 mm and no fetal pole or yolk sac can be identified on endovaginal scanning, then this suggests a failed pregnancy (anembryonic pregnancy).
In a normal early pregnancy, the diameter of the yolk sac should usually be <6 mm while its shape should be near spherical.
Visualisation of multiple yolk sacs is the earliest sign of a polyamniotic pregnancy, e.g twins.
Natural course
As the pregnancy advances, the yolk sac disappears and is often sonographically undetectable after 14-20 weeks.
Related pathology
-</ul><p>In general if the MSD is ≥ 25 mm and no fetal pole or yolk sac can be identified on endovaginal scanning, then this suggests a <a href="/articles/non-viable-pregnancy">failed pregnancy</a> (<a href="/articles/anembryonic-pregnancy">anembryonic pregnancy</a>). </p><p>In a normal early pregnancy, the diameter of the yolk sac should usually be <6 mm while its shape should be near spherical.</p><p>Visualisation of multiple yolk sacs is the earliest sign of a polyamniotic pregnancy, e.g twins.</p><h5>Natural course</h5><p>As the pregnancy advances, the yolk sac disappears and is often sonographically undetectable after 14-20 weeks.</p><h4>Related pathology</h4><ul>- +</ul><p>In general if the MSD is ≥25 mm and no fetal pole or yolk sac can be identified on endovaginal scanning, then this suggests a <a href="/articles/non-viable-pregnancy">failed pregnancy</a> (<a href="/articles/anembryonic-pregnancy">anembryonic pregnancy</a>). </p><p>In a normal early pregnancy, the diameter of the yolk sac should usually be <6 mm while its shape should be near spherical.</p><p>Visualisation of multiple yolk sacs is the earliest sign of a polyamniotic pregnancy, e.g twins.</p><h5>Natural course</h5><p>As the pregnancy advances, the yolk sac disappears and is often sonographically undetectable after 14-20 weeks.</p><h4>Related pathology</h4><ul>
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