The first part of a pregnancy to be visible on ultrasound is the “gestational sac” which can be seen with a transvaginal scan approximately 3 weeks after a baby is conceived. At this stage it appears empty. During the next 2 weeks first a “yolk sac” and then a small “fetal pole” become visible.
Because the time of conception is often uncertain, a dating scan may not show as advanced a pregnancy as expected based on the period dates. This does not necessarily imply that there is a problem, just that the dates may be different.
As part of the scan report the due date for the baby based on the ultrasound size in the first 12 weeks will be provided to your doctor. Each doctor has a different approach to whether period or ultrasound dates should be used, but in general except for women with regular cycles and sure period dates, a good ultrasound will provide a more accurate prediction of the due date.