It is important to note that in early pregnancy, a transvaginal ultrasound in much more accurate than an abdominal ultrasound, so we will talk about findings on a transvaginal ultrasound alone.
In viable pregnancies, a transvaginal ultrasound should be able to detect the gestational sac by 5 weeks gestational age. The gestational sac is usually the first sign of pregnancy on ultrasound and may be seen as early as 3 weeks. At this time the average diameter of the sac is 2 to 3 millimeters. At around 5.5 weeks the yolk sac often becomes visible inside the gestational sac.
When correlated with hCG levels, a gestational sac should be seen on ultrasound when the hCG level when it has reached about 1500 to 2000.