The yolk sac is one of the first structures observed during an early pregnancy ultrasound, often becoming visible around five weeks of gestation via a transvaginal scan. It appears as a small, spherical structure within the gestational sac. This temporary organ plays a fundamental role in supporting the embryo during the initial phase of development before the placenta fully forms and takes over necessary functions. Monitoring its characteristics, particularly its size, is a standard part of first-trimester prenatal care.
The Role of the Yolk Sac in Early Gestation
The yolk sac performs several biological functions that are crucial for the embryo’s survival in the first few weeks. Before the establishment of a fully functional placental circulation, the yolk sac acts as the primary source of nutrition. It transfers essential nutrients, including proteins, carbohydrates, and lipids, to the tiny embryo.
It also facilitates the exchange of gasses necessary for metabolic functions. A primary function is its role as the initial site of hematopoiesis, which is the formation of blood cells. The yolk sac produces the earliest forms of red and white blood cells until the liver and bone marrow take over this process. Furthermore, it helps create cells that contribute to the development of the umbilical cord and the gastrointestinal system.
Interpreting Normal Yolk Sac Measurements
The yolk sac is typically measured by its inner-to-inner diameter using high-resolution transvaginal ultrasonography. Its size is expected to increase progressively from the fifth week until approximately the tenth week of gestation. This measurement is used to help confirm gestational age and predict the viability of the pregnancy.
At about six weeks of gestation, a normal yolk sac typically measures around 2 to 3 millimeters (mm). Its diameter steadily increases over the next few weeks, generally remaining within a range of 3 mm to 5 mm. The maximum size is usually reached between 9 and 10 weeks, often peaking near 6 mm.
This steady, linear growth confirms the expected developmental trajectory of the early pregnancy. The measurement provides a reliable baseline for clinicians to compare against expected standards. A normally developing yolk sac should appear perfectly round with a clear, anechoic (fluid-filled) center and a distinct, echogenic (bright) wall.
Clinical Significance of Abnormal Size
Deviations from the expected size range or shape of the yolk sac can be an important indicator of potential pregnancy complications. A diameter of 6 millimeters or more, particularly before seven weeks of gestation, is often considered abnormally large. This enlargement is associated with a significantly increased risk of spontaneous miscarriage and fetal abnormalities.
Conversely, a yolk sac that is too small, such as less than 2 mm to 3 mm between 6 and 10 weeks, can also suggest an abnormal pregnancy. The absence of a visible yolk sac is concerning when the gestational sac reaches a mean diameter of 8 to 10 mm on a transvaginal scan, often indicating a failed pregnancy. A misshapen, irregular, or calcified yolk sac can be a predictor of a poor outcome, even more so than the absolute size alone.
When an abnormal size or shape is identified, a clinician will typically recommend a follow-up ultrasound examination one or two weeks later. This serial monitoring helps determine if the size discrepancy is a transient finding or an indicator of a non-viable pregnancy, such as an anembryonic pregnancy. In cases of an abnormally large yolk sac, the abrupt increase in diameter often precedes the actual pregnancy loss.
The Disappearance of the Yolk Sac
The yolk sac is a temporary structure designed to support the embryo until the placenta assumes its functions. Around the 10th week of gestation, as the placenta becomes fully established, the yolk sac begins its regression. Its size starts to shrink and it becomes less prominent on ultrasound images.
The yolk sac is eventually incorporated into the developing umbilical cord, where its remnant becomes known as the vitelline duct. By the end of the first trimester, typically around 12 to 14 weeks of gestation, the yolk sac is no longer visible or measurable on ultrasound. Its timely disappearance signifies a successful transition of supportive functions to the permanent placental structure.