What is the Difference Between Yolk Sac and Fetal Pole

The key difference between yolk sac and fetal pole is that the yolk sac is a structure that develops inside the uterus in early pregnancy and mainly provides nutrients to the embryo, while the fetal pole is one of the first stages of embryo development that normally develops into a fetus.

Pregnancy or gestation normally extends for 40 weeks and starts from the first day of the last menstrual period. It is classified into three stages known as trimesters. The yolk sac and fetal pole are present at the early stages of a healthy pregnancy. Both are located in the gestational sac in the uterus. They are contained inside the gestational sac in the uterus. Both the yolk sac and fetal pole can be viewed through an ultrasound. However, the yolk sac and fetal pole have different functions in pregnancy.

CONTENTS

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

What is Yolk Sac?

The Yolk sac is a small pouch-like structure made from thin membranes. It is one of the first things to develop during pregnancy and one of the first things to see on a prenatal ultrasound. The yolk sac begins to develop during the second week of gestation or pregnancy. The functions of the yolk sac may include circulating the gasses between the baby and the mother, creating the first form of blood cells, creating the cells that turn into other important structures, providing the baby with nutrients, and providing the baby with metabolism and immune function.

There are certain problems associated with the yolk sac. If the yolk sac is bigger than usual or missing, it results in a miscarriage. Furthermore, certain types of cancers, like yolk sac tumors or germ cell tumors, are also identified in the yolk sac.

What is Fetal Pole?

A fetal pole is also called an embryo. It is one of the initial phases of a healthy pregnancy. Moreover, it is normally the first stage of embryonic growth visible in an ultrasound. The fetal pole is usually identified at six weeks with vaginal ultrasound and at six and half weeks with abdominal ultrasound.

It is normally curved in appearance. The fetal pole also has a head and a tail-like structure. The distance between these two is known as crown-to-lump length, which helps to determine how far along mothers are in their pregnancy. The main function of the fetal pole is to develop into a fetus. Furthermore, conditions associated with the fetal pole are blighted ovum, miscarriage, and ectopic pregnancy. In blighted ovum and miscarriage, the fetal pole is missing, while in the case of ectopic pregnancy, the fetal pole is somewhere else other than the uterus.

What are the Similarities Between Yolk Sac and Fetal Pole?

• The Yolk sac and fetal pole are present at the early stages of a healthy pregnancy.
• They are located inside the gestational sac in the uterus.
• Both the yolk sac and fetal pole can be viewed through ultrasound.
• Also, both are associated with certain conditions.

What is the Difference Between Yolk Sac and Fetal Pole?

The Yolk sac is a structure that develops inside the uterus in early pregnancy and mainly provides nutrients to the embryo, while the fetal pole is one of the first stages of embryo development, which normally develops into a fetus. Thus, this is the key difference between yolk sac and fetal pole. Furthermore, the yolk sac appears in the second week of pregnancy. On the other hand, the fetal pole appears in the sixth week of pregnancy.

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

FAQ: Yolk Sac and Fetal Pole

When does a fetal pole develop?

The fetal pole becomes apparent at 6 weeks of gestation.

What is the distance between the fetal pole and the yolk sac?

The optimal cutoff value of the yolk sac-fetal pole distance is 80 mm.

What does a fetal pole look like?

A fetal pole is a curved structure. At one end, there is a head, and at the other end, there is a tail-like structure.

Summary – Yolk Sac vs. Fetal Pole

The Yolk sac and fetal pole are present at the early stages of a healthy pregnancy. The yolk sac is a small pouch-like structure made from thin membranes. It mainly provides nutrients to the embryo. On the other hand, the fetal pole is curved in appearance and has a head and tail-like structure. A fetal pole normally develops into a fetus. So, this is the summary of the difference between yolk sac and fetal pole.

Reference:

1. “Embryology, Yolk Sac.” Statpearls – NCBI Bookshelf.
2. “Fetal Pole: Ultrasound, Anatomy & Function.” Cleveland Clinic.

Image Courtesy:

1. “Human Yolk Sac from Tubal Pregnancy” By Ed Uthman (CC BY 2.0 DEED) via Flickr
2. “464 – US 1st trimester pregnancy” By Emergency Medicine Clinic (CC BY-NC-SA 2.0 DEED) via Flickr