- What is a Yolk Sac
- Why You Need a Yolk Sac
- And What is on Ultrasound?
- But What Happens Next to the Yolk Sac
- What Does it Mean When the Yolk Sac is Not Visible?
What is a Yolk Sac
The yolk sac is formed at the beginning of the third week from endoblastomic bubble (endoblast derivant). It does not function long-about three months, but is of great importance for the development of the fetus. That's why the sizes of yolk sac in the first six weeks after fertilization of the egg are much larger than the sizes of amniotic cavity together with the emerging embryo.
Why You Need a Yolk Sac
At the earliest stages of development, when the embryo is formed, only a yolk sac performs a variety of functions, without which normal development is impossible.
- From 18-th day of the beginning of a new life in the yolk sac the first embryonic red blood cells (erythroblasts) are formed and capillaries begin to grow from which later the entire fetal circulatory system is formed.
- From the 28th day the walls of yolk sac produce the first germ cells, which later will move to embryo gonads. It is important to note that at this time the eggs of a future girl are formed. If at this point the mother suffered or experienced severe stress-the formation of eggs will happen incorrectly and, in the future, an adult woman will suffer from infertility.
- Until the sixth week the yolk sac plays the role of "primary liver" and produces proteins which are very important for the development of the embryo such as alpha-fetoprotein.
The yolk sac is also actively involved in metabolic processes, the formation of immunity and gathers discharges of the fetus.
And What is on Ultrasound?
On the ultrasound the yolk sac is visible from 6-th till 12-th week of pregnancy. If the physician does not see the yolk sac, it is a very unfavorable sign: the pregnancy may stop to evolve at any moment. Also premature disappearance of the yolk sac is an unfavorable sign.
Normally, the yolk sac should have the dimensions:
- less than 5.5 mm at 5-10 weeks;
- more than 2 mm at 8-12 weeks.
But What Happens Next to the Yolk Sac
At the end of the first trimester the fetus is fully formed and goes on placental circulatory type. The yolk sac becomes redundant and is reduced, i.e. disappears. Until the end of pregnancy it remains in the form of small cysts at the base of the umbilical cord. If the yolk sac is reduced before it is due, when all the organs of the fetus have not yet been formed, the pregnancy will no longer develop. Just the embryo will not be able to move to a new stage of development and become a fetus. This is what often leads to the development of non-viable pregnancies and miscarriages.
What Does it Mean When the Yolk Sac is Not Visible?
The yolk sac is formed on the 15-16th day after conception and ceases to function at the end of the first trimester-11th week of pregnancy. The yolk sac plays an important role in breathing and nutrition of a fetus, performs the excretory and other functions before formed organs of the fetus and placenta assume this function. After 12-13th week, the yolk sac retracts into the cavity of the fetus, decreases and remains only in the form of cystic formation at the umbilical cord. The yolk sac normally becomes visible from the 6 to 12th week of pregnancy and is a rounded thin-section anechogenic formation in the immediate vicinity of the embryo. Its diameter is 4-8 mm (at the 7-th week of 4-5 mm), gradually increases to 10 mm, and then its reverse development begins.
If the yolk sac is not visible at the 6 to 11th week: Maybe the pregnancy have not been properly identified, and the pregnancy is less than 6 weeks (up to 6 weeks normally the yolk sac is not visible). Ultrasound can determine the heartbeat of the embryo (4-5 weeks) and the embryo is visible. - No yolk sac after 6 weeks can talk about adverse pregnancy, which may result in interruption of the pregnancy. Unfavorable prognosis. You must undergo an ultrasound again, preferably in transvaginal way with the high-resolution device, to verify the presence or absence of yolk sac. After the 12th week normally if the yolk sac is not identified. This means that the fetus has begun to receive nutrients through the placenta.
Video: What is the Pregnancy Yolk Sac