Pregnancy is a very important phase in the life of a woman which result in some physical changes, especially in appearance and in the reproductive organs. The hormones released during this time help in the growth of the fetus and general health of the woman. Yolk sac helps as a circulatory system for the human embryo until an internal system is developed.
In the gestational sac, the first structure which is visible is the yolk sac. It is better to see a yolk sac when the gestational sac has grown to measure over 10 mm. The normal size of the yolk sac is round and it measures approximately 6 mm. If the measure grows beyond 6 mm and displays bizarre shape or is calcified, it can be considered abnormal. Pregnancies with such abnormal yolk sac often fail.
The yolk-sac is located on embryo’s ventral side and is lined by entoderm. Outside it is the layer of mesoderm. The yolk sac is brimming with vitelline fluid which usually is used for nourishing the embryo at the start of the pregnancy.
One of the most important needs of any embryo throughout the pregnancy is regular supply of nutrition and blood without which the fetus cannot grow. This blood is initially transported to the sac wall by primitive aortae and after travelling through wide-meshed capillary plexus, it returns back via vitelline veins into the embryo’s tubular heart. This is called as the vitelline circulation and during this process, nutrition from the yolk sac is sent to the embryo.
The yolk sac appears like a pear shaped vesicle which is also termed as umbilical vesicle at the ending of fourth week and opens via long narrow tube called as vitelline duct into the digestive tube. This vesicle looks like a oval shaped small body with a diameter varying between 1mm to 5 mm after the birth of the child. It is situated in between the chorion and the amnion and lies at a varying distance from the placenta. The duct compulsorily undergoes total obliteration as the seventh week starts. Sometimes the proximal part of the duct continues as the diverticulum coming out from the small intestine. Meckel’s diverticulum is situated 3 to 4 feet above ileocolic junction and is usually attached to the abdominal wall of the umbilicus by fibrous cord.
Some times the ultrasound does not show the presence of yolk sac. In such cases, the doctors think that either the woman had a miscarriage or the pregnancy is still in very early stage. Generally, the yolk sac starts showing during the 5.5 weeks gestation period. If the yolk sac is not present during this time period, it can also mean that the woman has made some errors in recalling her last menstrual period and incorrect date can be the reason. A second ultrasound, which is usually done after some time, shows yolk sac along with the fetal pole.