Ultrasound assessment of fetal chorionicity. Biometry, anatomy, Doppler velocimetry, and amniotic fluid volume are used to identify and monitor twin pregnancies at risk of adverse outcomes such as twin-to-twin transfusion syndrome (TTTS) and fetal growth restriction. As in singletons, impaired fetal growth can be assessed in twins by comparing biometry and Doppler velocimetry parameters against standards for uncomplicated pregnancy. This chapter addresses the role of ultrasound in the care of uncomplicated and complicated twin pregnancies. It provides guidance on the methods used to determine gestational age and chorionicity, screening for chromosomal and structural abnormalities, and screening for TTTS, growth abnormalities, and preterm birth.