What is the Difference Between Yolk Sac and Gestational Sac

The key difference between yolk sac and gestational sac is that the yolk sac is the second structure that is identified in the uterus in early pregnancy, which is visible at approximately 5 and half weeks, while the gestational sac is the first structure that is identified in the uterus in early pregnancy which is visible at approximately 5 weeks.

The double bleb sign is an ultrasonographic feature where there is a visualization of the gestational sac containing the yolk sac and amniotic sac. This gives the appearance of two small bubbles. Moreover, the embryonic disc is located between the two bubbles. The embryonic disc is a part of the inner cell mass of the blastocysts from which the embryo of a mammal develops.

CONTENTS

1. Overview and Key Difference
2. What is Yolk Sac  
3. What is Gestational Sac
4. Similarities – Yolk Sac and Gestational Sac
5. Yolk Sac vs Gestational Sac in Tabular Form
6. Summary – Yolk Sac vs Gestational Sac

What is Yolk Sac?

The yolk sac is the second structure identified in the uterus in early pregnancy. It is visible at approximately 5 and half weeks of pregnancy. It is a membranous sac attached to an embryo that is formed by the cells of the hypoblast adjacent to the embryonic disc. The yolk sac is the first structure that is seen within the gestational sac during pregnancy. It develops in the gestational sac, usually at 3 days of gestation. Yolk sac generally provides an embryo with nourishments (food). It also helps circulate gasses between the mother and the embryo. Moreover, this sac produces cells that turn into important structures such as the umbilical cord, blood cells, and reproductive organs.

Figure 01: Yolk Sac

A normal healthy yolk sac can help to confirm a healthy early pregnancy. The yolk sac is a round or pear-shaped pouch with the size of 3 to 5 mm. It is made up of thin membranes. The early form of the embryo, which is known as the fetal pole, can be visible next to the yolk sac. Furthermore, the yolk sac gradually becomes smaller and disappears after 10^th week. Finally, it is absorbed by the developing embryo. Sometimes, some cancers can occur in the yolk sac and are known as yolk sac tumors. Yolk sac tumor is a germ cell tumor.

What is Gestational Sac?

The gestational sac is the first structure that is identified in the uterus in early pregnancy. It is visible at approximately 5 weeks of pregnancy. It is the large cavity of fluid surrounding the embryo. During embryogenesis gestational sac consists of the extraembryonic coelom. The gestational sac is spherical in shape and located in the upper part (fundus) of the uterus.

Figure 02: Gestational Sac

By nine weeks of gestational duration, the amniotic sac has expanded to occupy the majority of the volume of the gestational sac. This eventually reduces extraembryonic coelom to a thin layer between the amnion membrane and mesoderm. By this time, the gestational sac is simply called an amniotic sac. Furthermore, the gestational sac helps to determine if an intrauterine pregnancy exists until the embryo can be identified.

What are the Similarities Between Yolk Sac and Gestational Sac?

• The yolk sac and gestational sac are two structures helping in the confirmation of the pregnancy through ultrasound.
• Both structures appear in early pregnancies.
• They are pouch like structures.
• These structures have specific roles.

What is the Difference Between Yolk Sac and Gestational Sac?

The yolk sac is the second structure that is identified in the uterus in early pregnancy, which is visible at approximately 5 and half weeks, while the gestational sac is the first structure that is identified in the uterus in early pregnancy, which is visible at approximately 5 weeks. Thus, this is the key difference between yolk sac and gestational sac. Furthermore, the yolk sac is comparatively smaller than the gestational sac.

The below infographic presents the differences between yolk sac and gestational sac in tabular form for side by side comparison.

Summary – Yolk Sac vs Gestational Sac

The yolk sac and gestational sac are two types of sacs that develop during early embryogenesis. They are helping in confirmation of the pregnancy through ultrasound. The yolk sac is the second structure that is identified in the uterus in early pregnancy, which is visible at approximately 5 and half weeks, while the gestational sac is the first structure that is identified in the uterus in early pregnancy, which is visible at approximately 5 weeks. So, this summarizes the difference between yolk sac and gestational sac.

Reference:

1. “Yolk Sac in Early Pregnancy: Meaning & Function.” Cleveland Clinic.
2. “Gestational SAC.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “2907 Embroyonic Disc, Amniotic Cavity, Yolk Sac-02” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia
2. “2 mm gestational sac” By Luis Goncalves – Own work (CC BY-SA 3.0) via Commons Wikimedia

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