Ultrasound evaluation during the first trimester has many advantages, including accurate pregnancy dating, detection of early fetal demise (3% of first trimester examinations), aneuploidy screening ,exclusion of ectopic pregnancy and identification and characterization of twin pregnancies (2% of first trimester examinations).

Nuchal translucency (NT) measurement with the addition of maternal age and maternal serum biochemistry is now a well established screening method which can detect approximately 90% of trisomy 21 fetuses at a 5% false positive rate.

Research is now directed at reducing the false positive rate by including additional first trimester screening parameters such as nasal bone assessment, Doppler assessment of the tricuspid valve and ductus venosus and fronto-maxillary facial angle assessment in the risk algorithm,These however may be difficult to perform and interpret , therefore appropriate training and ongoing audit is essential.

With improved resolution of ultrasound there is now greater emphasis on screening for structural abnormalities in the first trimester. Studies have shown that up to two-thirds of fetal structural abnormalities may be identified in the first trimester. This is dependent on the organ system involved.

Evaluation of placentation should be considered in the first trimester.This should incorporate cord insertion as well as thickness and morphology.

The first trimester is the ideal time to establish the chorionicity in twin pregnancies as it becomes harder to clarify at later gestations. This information influences the clinical management of the pregnancy.