The yolk sac: what it is, why and how it forms

The yolk sac is the first structure in which the embryo develops within the gestational chamber . When detected through an ultrasound, it is the first sign of a pregnancy.

What is the yolk sac

The yolk sac (or umbilical vesicle ) is an annex of the embryo which contains the calf (the set of substances necessary for the formation of the embryo and metabolised by it during its development).

This is a small pear-shaped sac that grows about a millimeter each week in the gestational sac (or chamber) inside the uterine cavity.

Inside there is a fluid (called vitelline fluid ) intended for nourishment of the embryo during the first phase of its development.

  • It begins to form a couple of weeks after the implantation of the embryo, and then progressively grow week after week.
  • At the end of the fourth week, this sac presents itself with a small opening which – via a connecting tube between the yolk sac and the lumen of the intestine called the vitelline duct – opens into the digestive tract.
  • It grows until about the 9th / 10th week of pregnancy, reaching a size of about five / six centimeters. Then it begins to gradually degenerate.
  • It disappears when the placenta is ready: at this point the unborn child will no longer be an embryo but a fetus (complete with all organs.

In the first weeks of pregnancy – to be precise towards the fourth / fifth week following the last menstrual cycle – the yolk sac is the first element that can be manifested by ultrasound inside the management room. In fact, it appears before the embryo, which is generally visible only after the sixth week of pregnancy .

Since the yolk sac is the first structure intended to house the new developing life, being able to detect it early in pregnancy via an ultrasound,

  • it is very useful for the doctor to check how this is progressing, as the size and shape of the yolk sac can indicate the existence of some anomaly;
  • but very often it is also the most reliable confirmation of a positive result of a pregnancy test.

The circulation within the yolk sac

As we have said, the function of the yolk sac is to nourish the embryo during the first weeks of gestation. The maternal blood passes through the vitelline veins (present in the yolk sac) from the mother’s primitive aorta to the heart of the embryo.

This mechanism is known as vitelline circulation , and allows the embryo to receive everything it needs to develop and grow properly.

In fact, it is precisely in these weeks of life that, feeding directly from the mother’s blood, the embryo begins to form in all its parts: from the muscles, to the bones, to the internal organs. Around the 6th week of pregnancy, her eyes form, and her heartbeat can be felt.

About two weeks after the implantation of the embryo in the uterus, a transitional structure is formed in the woman’s body: the primary yolk sac . This structure will then remodel itself in the secondary yolk sac , and then remodel itself towards the fourth week in the definitive one .

With the definitive yolk sac, the transformation and evolution of this element ends: its development will continue for a few weeks. In this phase the yolk sac reaches its final shape and size, which will remain so until about the tenth / eleventh week and then gradually regress.

The importance of the yolk sac in follow-up ultrasounds

When a pregnancy test reveals a positive result , the woman is usually given an initial ultrasound to check that the embryo has implanted correctly inside the uterus – rather than in extrauterine cavities. But also to have other information of considerable importance.

During this first ultrasound, it can sometimes happen that the yolk sac is visible, but not the embryo. This result may be due to various reasons: for example, it may be that fertilization took place later than expected, or that a miscarriage occurred already in the first few weeks.

In fact, it is necessary to take into account the fact that fertilization does not necessarily occur immediately after sexual intercourse, since the spermatozoa survive several days inside the uterus. A second ultrasound performed a few days later can in fact easily reveal an already very different situation.

On the other hand, when both the gestational chamber and the yolk sac appear clearly in the ultrasound, but the embryo has not yet developed, it could be the case, as we have said, of a spontaneous abortion. Such an early abortion is by no means rare, and often constitutes the physiological resolution operated by the organism in relation to abnormal cells and possible malformations of the fetus.

Although it is not a fixed rule, even the size of the yolk sac can somehow reveal anomalies in the pregnancy.

  • Pregnancies that do not progress are often characterized by a larger than normal yolk sac.
  • When, on the other hand, the yolk sac is very small, the doctor usually prescribes – for a few days later – a second ultrasound to check that the situation is normalizing or for further investigations.

Similarly, an excessive accumulation of liquids inside the yolk sac can also indicate an abnormal development of the embryo or a miscarriage. If, on the other hand, the ultrasound shows two yolk sacs, this can often indicate that it is a twin birth, a birth which at the same time can present a single or double gestational chamber.

The disappearance of the yolk sac and the development of the placenta

After the 10th week , the yolk sac begins to decrease in size, only to disappear completely within a couple of weeks when the placenta is complete. From that moment on, the embryo transformed into a fetus will receive the mother’s nourishments through the placenta via the umbilical cord.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform, Maternicity.com.

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