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Sonographic evaluation of abnormalities in yolk sac

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by

Ahmed Okasha

on 30 October 2014

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Transcript of Sonographic evaluation of abnormalities in yolk sac

Sonographic evaluation of abnormalities in yolk sac

Introduction
secondary yolk sac is the
first
extra-embryonic structure that becomes
sonographically visible
within the gestational sac

the yolk sac is the
primary route of exchange
between the embryo and the mother.

provides nutritional, immunologic, metabolic, endocrine, and hematopoietic
functions until the placental circulation is established

one of the most important conceptional structures evaluated sonographically
in the first trimester

is
confirmation
of an intrauterine pregnancy

sonographic features related to the shape, size, and internal structure of the yolk sac can be
associated with the gestational outcome

The yolk sac should always be
visualized when the mid gestational sac diameter is 6-8mm.

Stages of the yolk sac and embryonic development
At the fourth week of embryologic development
the wall of the yolk sac consists of 3 thin cellular layers.
The outermost layer is the ectoderm, which faces the exocoelomic cavity it consists of a distinct layer of flattened cells.
The innermost layer facing the yolk sac cavity is the endodermal epithelium, which is composed of a single layer of cuboidal epithelial cells.
Located between these two layers is the mesodermal layer, which is a very narrow tissue, consists of blood island formations in which hematopoietic stem cells can be identified throughout a primitive capillary network.
By the end of the fourth gestational week
primitive blood cells are widely scattered in embryonic blood vessels located in the primordium of the heart, mesonephros, and other embryonic organs.
From the fifth gestational week onward
two compartments are clearly distinguished in the wall of the yolk sac. The mesodermal & the endodermal compartments.
After the seventh week of embryologic development
the signs of regression begin on the wall of the yolk sac.

The yolk sac is connected to the embryo by the vitelline duct. Normally, the yolk sac appears as a circular structure with an anechoic center surrounded by a uniform well-defined echogenic wall. Usually the inner diameter of a yolk sac measures 3 to 5 mm. In fact, the yolk sac size progressively increases from the beginning of the 5th gestational week to the end of the 10th gestational week. Afterward, the yolk sac size decreases gradually.

The number of yolk sacs present in a gestational sac can aid in determining the amnionicity of the pregnancy. Thus, there will be 2 embryos, 1 chorionic sac, 1 amniotic sac, and 1 yolk sac in a monochorionic monoamniotic twin pregnancy.
Normal Yolk Sac
Abnormalities in Yolk Sac
Large Yolk Sac
- The upper limit for the size of a normal yolk sac in pregnancies with a gestational age from the 5th to the 10th weeks is 5 or 6 mm.

- A yolk sac diameter of greater than 5 mm is associated with an increased risk of spontaneous abortion.

- It has been suggested that an abnormally large yolk sac may indicate a poor obstetric outcome; therefore, close follow-up with sonography is recommended for these pregnancies.
Abnormalities in Yolk Sac
Small Yolk Sac
- A yolk sac diameter of 2 mm or less may be associated with an adverse outcome in pregnancies with a gestational age of 5 to 8 weeks.

- It is well known that the yolk sac size begins to decrease during the late weeks of the first trimester, so gestational age should be taken into account when the size of the yolk sac is assessed.

Abnormalities in Yolk Sac
Calcified Yolk Sac
-A calcified yolk sac may appear as an echogenic ring on sonography. It has not been reported to be associated with a live embryo before the 12th week of gestation.

– A calcified yolk sac would be observed only with a dead embryo because the yolk sac would undergo calcification within a few days after embryonic death has occurred.
Abnormalities in Yolk Sac
Echogenic Yolk Sac
- An echogenic yolk sac is different from a calcified yolk sac in that echogenicity may represent various types of materials other than calcium. However, tissue harmonic imaging may be more useful for confirmation of this finding.

-An echogenic yolk sac can be associated with fetal death or abnormalities. It is usually associated with aneuploidy.
Abnormalities in Yolk Sac
Absence of the Yolk Sac
-The yolk sac performs important functions for embryonic development during organogenesis.

-On transvaginal sonography, absence of the yolk sac in the presence of an embryo is always abnormal and in general is associated with subsequent embryonic death.

Abnormalities in Yolk Sac
Irregular Yolk Sac Shape
-Yolk sacs with mainly wrinkled margins, indented walls, or both are usually identified as having an irregular shape.

-There are a number of clinical studies that have declared that the persistence of an irregular yolk sac shape may be used to indicate an adverse gestational outcome.
Abnormalities in Yolk Sac
Persistent Yolk Sac
- When the 10th or 11th week of gestation is completed, the yolk sac begins to shrink rapidly and eventually disappears.

- However, sometimes the yolk sac can persist between the amnion and the chorion even after 12 weeks’ gestation. The persistent yolk sac usually stands close to the site where the umbilical cord enters the placenta. They may sometimes persist even to the 13th week of gestation. The persistence of the yolk sac seems to be unrelated to an adverse perinatal outcome.
Abnormalities in Yolk Sac
Floating yolk sac
Yolk sac which is moving within the gestational sac, is usually associated with adverse pregnancy outcome.

Conclusion
Assessment of the yolk sac should be part of a complete first-trimester sonographic examination.
An abnormality in the sonographic appearance of a yolk sac can predict subsequent embryonic death or abnormalities.
Therefore, accurate recognition of the normal and abnormal sonographic findings related to the yolk sac can be used to anticipate the course of pregnancy.
Thank You
vitelline duct
Full transcript