Yolk sac: what it is and at what week of pregnancy it is possible to see it on ultrasound
The yolk sac is the first structure that forms inside the gestational chamber during pregnancy, visible at the first ultrasound, ie starting from the fifth week of gestation. The yolk sac contains, protects and nourishes the embryo inside it. gestational chamber and it is very important for its development. Before discovering together everything there is to know about the yolk sac, here is a video that explains what are the different ultrasound scans to do during pregnancy:
What is the yolk sac?
The yolk sac (also known as "yolk sac") is an external membrane with respect to the embryo, very useful precisely for its development during pregnancy. The yolk sac, in fact, supplies the embryo itself with nourishment and gives rise to the first cells blood and the first germ cells, from which the fetus can develop.
The yolk sac originates two weeks after the embryo is implanted in the uterus. It will be possible to see it during the first pregnancy ultrasound, which usually takes place around the fifth week of gestation. The growth of the yolk sac is continuous, at least until the tenth week of gestation, when it gradually begins to shrink. By the twelfth week it will be completely gone, because it is reabsorbed by the placenta.
If two yolk sacs appear in place of one yolk sac during the first pregnancy ultrasound, it means that yours is a twin pregnancy. Other times, however, it may happen that the yolk sac is not yet visible inside the gestational chamber at the fifth week of gestation, but this need not worry.
When it becomes visible, the yolk sac consists of a sort of pear-shaped bladder, opened in the digestive tract of the embryo through a duct, the so-called "ductus calf." The diameter of the yolk sac around the fifth week of gestation is about 2 mm and will grow until it reaches 5.5 mm at the tenth week. At this point its natural deterioration will begin, and this is because the yolk sac will no longer be necessary: there will be other organs and structures (first of all the placenta) to perform its function.
See also
Primary, secondary and definitive yolk sac: how it develops
The yolk sac goes through three different stages of development and, depending on the stage in which it is located, it will be called the primary yolk sac, the secondary yolk sac or the definitive yolk sac. The primary yolk sac is that of the initial phase and is formed starting from the second week of the embryo's life.
The secondary yolk sac, on the other hand, represents the second phase of development, in which it gradually changes its shape, until it becomes definitive during the fourth week of gestation: at this point it has assumed its final shape and presents in the upper part the intestinal tube and in the lower one the so-called "ventral mesentery", which connects the stomach and the upper part of the loop of the duodenum to the anterior abdominal wall of the embryo.
Empty calf sac: what does it mean?
During the first ultrasound of pregnancy it may happen to the specialist to identify the yolk sac, but not the embryo that should be contained inside it. In these cases we will speak of "empty yolk sac". The reasons that can cause an empty yolk sac are different, first of all - unfortunately - a spontaneous abortion. The "spontaneous abortion is very common in the first weeks of gestation and often represents a sort of" natural selection ", so - if there are malformations in the cells of the fetus or another kind of problem - the embryo eliminates itself.
But an empty yolk sac could also be caused by other reasons, first of all a delay in ovulation and fertilization compared to what was expected. If fertilization took place late, it will simply be still early to see the embryo and detect traces of it. cardiac activity. A second ultrasound will need to be scheduled a few days later to make sure the sac is actually empty, but in the meantime it will be good to perform the Beta hCG test. The levels of the Beta hCG hormone, also called pregnancy hormone, will be able to indicate whether or not a miscarriage occurred.
Irregularity of the yolk sac
Ultrasound may also detect the presence of irregularities in the yolk sac. If, for example, the yolk sac is found to be very swollen with fluid, it is referred to as a "dropsy yolk sac", a condition often linked to a miscarriage.
If the yolk sac, on the other hand, has a very large size, being larger than it should appear in that specific week, it could indicate the presence of problems related to chromosomal risks or to some particular syndrome. In some cases it could also be an indication of spontaneous abortion. It will be the gynecologist's care to repeat the transvaginal ultrasound after a few days in order to better monitor the situation and give a correct diagnosis.
For more scientific information on the yolk sac, you can consult the MED 2000 website.
