The 6-week ultrasound is often the first opportunity to visualize the developing pregnancy and is frequently referred to as a viability or dating scan. This early appointment is based on the calculation of six weeks from the first day of the last menstrual period (LMP). Because the embryo is still extremely small at this stage, the scan is typically performed using the transvaginal ultrasound method for the clearest possible images. This approach involves inserting a small probe into the vagina, positioning it closer to the uterus, which provides a higher resolution view of the tiny structures within the gestational sac. The primary goal of this initial examination is to confirm that the pregnancy is located inside the uterus and is developing along the expected timeline.
The Essential Structures Visible
At six weeks gestation, the ultrasound should clearly identify three distinct structures that confirm the presence of an intrauterine pregnancy.
The first structure is the gestational sac, which appears as a small, dark, fluid-filled space surrounded by a bright white ring of tissue inside the uterus. This sac houses the developing embryo and is the earliest sonographic sign of pregnancy, typically measuring around 10 to 15 millimeters in diameter at this point. The presence of the sac alone confirms the location of the pregnancy but requires further visual evidence to confirm viability.
Within the gestational sac, the yolk sac is the next structure that should be identified, appearing as a small, thin-rimmed circle. The yolk sac is an important landmark because it provides nourishment to the embryo before the placenta is fully functional. At six weeks, this structure is usually visible adjacent to the developing embryo and typically measures less than 6 millimeters across.
Finally, the fetal pole represents the first visual sign of the embryo itself and is often visible as a small, thickened area adjacent to the yolk sac. This structure, which will eventually develop into the fetus, is typically seen as a tiny, featureless, echogenic speck on the scan. At this early point, the fetal pole is still very small, often measuring only 2 to 5 millimeters in length.
Key Measurements for Viability
Beyond simply identifying the structures, the sonographer takes precise measurements to determine the gestational age and assess the likelihood of the pregnancy continuing.
The most important metric obtained at this stage is the Crown-Rump Length (CRL), which measures the embryo from the top of its head (crown) to the bottom of its torso (rump). This measurement is considered the most accurate method for dating the pregnancy during the first trimester, offering an estimated due date with a margin of error of only a few days.
At exactly six weeks gestation, the CRL is expected to be very small, generally falling within a range of approximately 2 to 5 millimeters. The embryo grows rapidly, approximately 1 millimeter per day during this period, which makes the CRL a highly reliable indicator of gestational age. If the embryo is measured to be slightly larger or smaller, the calculated gestational age may be adjusted, which often explains minor differences from the LMP-based dating.
The second significant measurement is the Fetal Heart Rate (FHR), which is documented once a rhythmic flicker of cardiac activity is observed. Although the heart begins to beat very early, it becomes reliably visible on an ultrasound around six weeks gestation. The expected heart rate for a healthy pregnancy at this time typically falls between 100 and 120 beats per minute (bpm). Observing a heart rate within this range provides strong initial confirmation of the pregnancy’s viability.
Addressing Common Variations and Next Steps
It is common for a 6-week ultrasound to yield results that are slightly different from what was initially expected, which can often cause worry. The most frequent reason for this variation is a simple discrepancy in the dating, where the pregnancy is not actually six weeks along. This occurs because the initial dating is based on the last menstrual period, which assumes ovulation happened exactly two weeks later, but variations in the menstrual cycle can result in the pregnancy being a few days earlier than calculated.
If the scan is performed a few days too early, the sonographer might only see a gestational sac and a yolk sac, but no definitive fetal pole or heartbeat. This is referred to as an inconclusive scan, meaning the viability cannot yet be confirmed. In this scenario, the healthcare provider will almost always recommend a repeat scan to be performed in 7 to 10 days.
This waiting period allows time for the expected growth to occur, ensuring that a potentially healthy but slightly younger pregnancy is not mistakenly diagnosed as non-viable. The transvaginal ultrasound remains the method of choice for the follow-up scan, as its high clarity is necessary to track the millimeter-scale growth of the embryo. A clear follow-up scan showing appropriate growth and a measurable heartbeat is generally sufficient to resolve the initial uncertainty.