The yolk sac is a temporary, specialized structure within the gestational sac, identified during an early ultrasound. This small, fluid-filled pouch is one of the first structures to develop after conception, appearing almost immediately after the fertilized egg implants in the uterus. The yolk sac confirms the pregnancy is located inside the uterus and functions as a provisional organ before the placenta and other systems are fully established. Understanding the presence, appearance, and size of the yolk sac provides medical professionals with important information about the health and viability of a developing pregnancy.
The Essential Function of the Yolk Sac
The yolk sac serves as the embryo’s primary source of nutrients, performing functions that the developing organs cannot yet manage. Before the full formation of the placenta and a functioning umbilical cord, it facilitates the transfer of essential substances like proteins, carbohydrates, and fats from the pregnant person’s circulation to the developing cells.
The yolk sac also functions as the first site of hematopoiesis, which is the formation of blood cells. During the early weeks of gestation, the sac produces the first red and white blood cells that circulate within the embryo. It also plays a role in the formation of the primitive gut and is involved in the development of the early circulatory system. The yolk sac grows until approximately the tenth week of gestation, after which the placenta takes over its functions, and the sac gradually shrinks and is absorbed.
When the Yolk Sac Appears on Ultrasound
The visualization of the yolk sac occurs during the first-trimester ultrasound, typically performed with a transvaginal probe for greater detail. It is usually the first structure identified inside the gestational sac, appearing at approximately 5 to 5.5 weeks of gestation. The sac presents on the screen as a distinct, round structure with an echo-bright (white) rim and an anechoic (black) center.
The presence of a yolk sac confirms an intrauterine pregnancy and is a necessary step before the appearance of the fetal pole, which is the earliest visible evidence of the embryo itself. In a typical pregnancy, the yolk sac measures between 2 and 5 millimeters in diameter. The structure will slowly increase in size until about the tenth week, after which it begins its regression as the embryo grows.
Interpreting Abnormal Yolk Sac Findings
Deviations from the expected size, shape, or presence of the yolk sac are associated with a reduced chance of a continuing, healthy pregnancy. An absent yolk sac, when the gestational sac has reached a mean diameter of 12 millimeters or more on a transvaginal ultrasound, suggests a non-viable pregnancy. This finding, sometimes referred to as a blighted ovum or an anembryonic pregnancy, means that a gestational sac has formed but the embryo has failed to develop.
The finding of an abnormally large yolk sac is also a cause for concern, often defined as a diameter greater than 5 or 6 millimeters. An enlarged yolk sac, particularly when seen before the seventh week of gestation, is correlated with a higher risk of spontaneous miscarriage. Conversely, an abnormally small yolk sac, typically measuring less than 2 millimeters, can also indicate a poor prognosis and an increased risk of early pregnancy loss.
Irregularities in the shape of the yolk sac, such as a distorted or non-spherical appearance, may also suggest an increased risk of an adverse outcome. Furthermore, the visualization of a double yolk sac is the earliest sign of a multiple gestation, indicating the presence of twins. The interpretation of any abnormal finding requires careful consideration of the gestational age and the measurement of other structures, such as the fetal pole and its heart rate.
Clinical Follow-Up and Management
When an ultrasound reveals a concerning yolk sac finding, the initial management involves careful observation and serial monitoring. Healthcare providers do not make a definitive diagnosis of non-viability based on a single early scan. Instead, the focus shifts to documenting the progression of the pregnancy over several days.
A repeat transvaginal ultrasound is scheduled seven to ten days after the initial scan to check for developmental milestones. This follow-up examination specifically looks for the appearance of the fetal pole and, most importantly, the presence of a fetal heartbeat. Serial measurements of human chorionic gonadotropin (hCG) hormone levels in the blood may also be used to track the viability of the pregnancy. The goal of this waiting period is to allow time for the embryo to develop so that an accurate prognosis can be established.