Ultrasound in Pregnancy

The 11 - 13 week first trimester scan
As we approach the end of the first trimester the developing embryo has now become a fetus, attaining the form and shape of a small human (12wk1). At this stage of pregnancy we can now discover much about a pregnancy:

1. Confirm viability - the majority of miscarriages will have taken place by the end of the first trimester. If the fetus appears healthy at this stage of the pregnancy, the risk of subsequent miscarriage is very low (~1%). In a small number of pregnancies a missed miscarriage may be found (the pregnancy is not viable but the placenta or afterbirth continues to be active, having missed the fact)

2. Confirm gestational age. When we count the number of weeks pregnant from the first day of the last period, we assume that conception took place exactly two weeks later. This is often not the case. The ultrasound measurement provides a more accurate estimate of gestational age, though we usually don't redate unless there is more than a week in the difference between the menstrual and ultrasound dates.

3. Confirm number of fetuses - This is the best time to check for twins and for the chorionicity of the twins (see twins and more).
(Continued next page)