The vitelline vesicle is the first structure seen on ultrasound within the gestational sac, usually on the first ultrasound of pregnancy. The vitelline vesicle is a crucial element in the early life of the embryo since it is the main route of exchange between the embryo and the mother.
Specialists believe that ultrasound characteristics related to the shape, size and internal structure of the yolk vesicle may be associated with the viability of pregnancy.
The vitelline vesicle is easily detected on transvaginal ultrasound when the mean diameter of the gestational sac is 5 to 6 mm. And it is generally accepted that the vitelline vesicle should be seen when the gestational sac measures more than 8 mm (Ultrasonographic prediction of early miscarriage.Hum. Reprod. (2011) 26 (7): 1685-1692)
Usually, the yolk vesicle appears as a white circle with a black center. Normally, the inner diameter of the vitelline vesicle is 3 to 5 mm. In fact, the size of the vitelline vesicle increases progressively from the beginning of the fifth gestational week to the end of the tenth gestational week. The size of the vitelline vesicle then gradually diminishes.
Absence of the vitelline vesicle on ultrasound
In transvaginal ultrasound, the absence of the yolk sac in the presence of an embryo is always abnormal and is generally associated with later embryonic death. It is rare to see a vitelline vesicle and an empty amniotic sac without an embryo.
Large vitelline vesicle on ultrasound
Although there is no consensus, most specialists accept a diameter of 5 or 6 mm as the maximum for the normal size of a vitelline vesicle in pregnancies between the fifth and tenth weeks. Size are associated with an increased risk of spontaneous abortion. However, some studies have mentioned the existence of a large vitelline vesicle in viable pregnancies.
Small vitelline vesicle on ultrasound
There is only one study that mentions that a yolk vesicle less than 2 mm in size between the eighth and 12th week of pregnancy may be indicative of miscarriage. A small vitelline vesicle can be seen very early in pregnancy or, as of week 11 of pregnancy.
Calcified vitelline vesicle on ultrasound
The vitelline vesicle is seen on the ultrasound as an intense white ring. It appears that the calcification of the vitelline vesicle occurs once the embryo has died.
Vitelline vesicle irregularly on ultrasound
A vitelline vesicle with wrinkled or jagged edges is classified as an irregularly shaped vitelline vesicle. There are a number of clinical studies that have stated that the persistence of an irregular form of the vitelline vesicle can be used to indicate abortion, but others find no relationship between irregular shape and an increased risk of miscarriage.
Persistent vitelline vesicle on ultrasound
When week 10 or 11 of gestation ends, the vitelline vesicle begins to shrink rapidly and eventually disappears. However, sometimes the vitelline vesicle may persist, even after 12 weeks of gestation, near the site where the umbilical cord enters the placenta. The clinical significance of a persistent persistent vitelline vesicle is unknown.