Ultrasound is a valuable diagnostic test throughout the first trimester of pregnancy. Early in this trimester, it is used to distinguish between normal intrauterine, failed intrauterine, and ectopic pregnancies. Later it can be used with maternal blood tests to screen for trisomy 21 and other forms of aneuploidy, and in some centers to assess fetal anatomy and diagnose structural anomalies. First trimester sonography is also useful for accurate assessment of gestational age. This article reviews these applications, the approach to establishing diagnoses, and ways to avoid diagnostic mistakes that can lead to serious errors in patient management and adverse pregnancy outcome.