The gestational sac is the first structure visible on an ultrasound, confirming a pregnancy is developing inside the uterus. This fluid-filled structure provides a protective space for the developing embryo. Identifying the sac is a crucial initial step for healthcare providers, as its presence in the uterus helps rule out an ectopic pregnancy. On the screen, it appears as a small, dark, rounded area, making it the earliest visual evidence of an intrauterine pregnancy.
When and How It Appears on Ultrasound
The gestational sac is typically first viewed using a transvaginal ultrasound, which provides a clearer image than an abdominal scan in early pregnancy. It usually becomes visible between 4.5 and 5 weeks of gestational age.
At this stage, the sac is only about 2 to 3 millimeters in diameter. Visualization often corresponds to a specific range of the pregnancy hormone human chorionic gonadotropin (hCG) in the bloodstream. While the threshold for detection can be low, the sac is consistently visible on a transvaginal scan once hCG levels reach approximately 1,500 to 2,000 mIU/mL.
Visual Characteristics of a Healthy Gestational Sac
A healthy gestational sac appears on ultrasound as a distinct, fluid-filled circle or oval. The fluid inside the sac registers as black or anechoic on the ultrasound image. This dark space is surrounded by a bright white, thickened rim of tissue, which represents the early placenta and the decidual reaction of the uterine lining.
This bright rim is often described as the “double decidual sac sign” (DDS), which is a definitive indicator of a true intrauterine pregnancy. The double ring is formed by two concentric echogenic (bright) layers. A normal sac will have a smooth, well-defined border and be eccentrically located within the endometrium. The size of the sac, known as the Mean Sac Diameter (MSD), is measured and correlated with the expected gestational age.
Structures Found Inside the Sac
The gestational sac is initially empty, but internal structures appear in a predictable sequence as the pregnancy progresses. The first structure to emerge within the sac is the yolk sac, which is typically visible around 5 to 5.5 weeks of gestation. On the screen, the yolk sac looks like a small, bright, thin-rimmed circular structure inside the larger, dark gestational sac.
The yolk sac provides nutrients to the embryo before the placenta takes over. Shortly after the yolk sac appears, the embryo itself becomes visible, often referred to as the fetal pole. This structure is seen as a tiny, thickened area adjacent to the yolk sac, usually around 6 weeks of gestation. Once the embryo is visible, its length, known as the Crown-Rump Length (CRL), becomes the most accurate way to date the pregnancy.
What Atypical Appearances May Indicate
Deviations from the expected appearance of the gestational sac can raise concerns about the viability of the pregnancy. An irregular, collapsed, or misshapen gestational sac border may suggest a miscarriage. If the sac wall appears extremely thin, it can also be a source of concern.
One common finding is the “empty sac” or anembryonic pregnancy, where the gestational sac grows but lacks the expected yolk sac and embryo. A gestational sac measuring 25 millimeters or more without a visible embryo is diagnostic of an early pregnancy loss. Furthermore, a sac that is too small relative to the embryo’s size (where the difference between the MSD and the CRL is less than 5 millimeters) is considered a sign of poor prognosis. Abnormalities with internal structures, such as a yolk sac that is too large (over 6 millimeters) or misshapen, are also associated with an increased risk of miscarriage.