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.
When the fetal pole grows to more than 4mm the baby’s heart beat can be picked up on the ultrasound and these things normally occur 4 ½ weeks after conception (about 6 ½ weeks from the last menstrual period).
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.
If the pregnancy is very early it may be normal not to clearly see the fetal pole or heart beat. In this case a follow up scan after an interval when normal growth can be expected to now show these things will be recommended.
In a pregnancy without abnormal bleeding, once the fetal heartbeat is seen on ultrasound there is only a 3% ongoing chance of miscarriage in that pregnancy.
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.