The yolk sac is a transient structure that forms early in human development, appearing as one of the first features visible on a prenatal ultrasound. It is a small, round membrane situated within the gestational sac, the fluid-filled cavity surrounding the developing embryo. This temporary structure serves its purpose during the embryonic period before other organs take over its responsibilities. Its appearance confirms an intrauterine pregnancy and its characteristics are important markers for monitoring the earliest stages of gestation.
The Essential Role of the Yolk Sac
Before the placenta is fully developed, the yolk sac acts as a temporary support system for the embryo. Its primary function is to facilitate the transfer of nutrients, including proteins, carbohydrates, and lipids, to the growing embryo through a primitive circulatory system.
The yolk sac is also the initial site for hematopoiesis (blood cell formation). It produces the first hematopoietic stem cells, which later migrate to the liver and bone marrow. Additionally, part of the yolk sac is incorporated into the embryo to form the primitive gut tube, the foundation for the entire gastrointestinal tract.
The Timeline of Appearance and Regression
The yolk sac becomes visible on a transvaginal ultrasound at approximately 5 to 5.5 weeks. It appears as a small, thin-rimmed circular structure inside the gestational sac. Its visualization often precedes the embryo itself, which typically becomes detectable around six weeks.
The yolk sac steadily increases in size, reaching its maximum diameter between 6 and 8 weeks. During this period, a normal yolk sac typically measures between 3 and 5 millimeters across its inner rim. Its size and shape are monitored because they provide early indications of the pregnancy’s progression.
Regression begins around 10 weeks as the placenta matures and the embryo develops further. It starts to shrink as its functions are gradually taken over by other structures. The yolk sac usually disappears from ultrasound view entirely by the end of the first trimester, typically around 12 to 14 weeks, marking the end of its functional role.
Incorporation and Fate of the Yolk Sac
The physical process by which the yolk sac disappears is known as involution, involving shrinking and eventual incorporation. As the embryo folds and grows, a portion of the yolk sac is drawn into the embryonic body, contributing to the formation of the midgut. The yolk sac remains connected to the midgut via the vitelline duct (or yolk stalk).
Normally, the vitelline duct completely closes and is absorbed by the developing fetus, usually by the twentieth week. The majority of the extra-embryonic part of the yolk sac shrinks and is no longer visible on ultrasound. In rare instances, a small remnant of the duct may persist as a pouch attached to the small intestine, known as Meckel’s diverticulum.
Clinical Significance of Yolk Sac Characteristics
The presence, size, and appearance of the yolk sac are important diagnostic markers used during early pregnancy monitoring. A normal, round yolk sac of the expected size is a reassuring sign of a viable intrauterine pregnancy. Its absence when the gestational sac is larger than 20 millimeters can suggest a non-viable pregnancy, such as a blighted ovum.
Abnormalities in the yolk sac’s size or shape are associated with adverse outcomes. A diameter greater than 5 or 6 millimeters between 5 and 10 weeks is often called a megayolk sac and indicates a higher risk of miscarriage or fetal anomalies. Conversely, a yolk sac that is too small for the gestational age is also a sign that the pregnancy may not be progressing.
When an abnormal yolk sac is detected, it prompts closer monitoring. Follow-up ultrasounds are recommended within seven to fourteen days to check for viability, growth of the embryo, and the presence of a fetal heartbeat. The visualization of the yolk sac helps providers assess the health and progression of the earliest stages of gestation.