Guidelines for the performance of the first trimester ultrasound.

The purpose of this document is to guide ultrasound practitioners in providing accurate information on the assessment of gestational age, viability and fetal development in the first trimester. In the presence of twins and higher order multiple pregnancies, it is also intended to assess chorionicity and amnionicity which have implications for risk assessment and continuing antenatal care. This guideline has been adopted by the ASUM Council and is applicable to all ultrasound practitioners.