This document provides guidelines from the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) on the role of ultrasound in twin pregnancies. It recommends that twin pregnancies be dated using the larger crown-rump length between 11-13 weeks of gestation. It also recommends determining chorionicity and amnionicity before 14 weeks using membrane thickness and placental mass number. Uncomplicated dichorionic twins should have scans at 11-14 weeks, 20-22 weeks, and every 4 weeks thereafter, while uncomplicated monochorionic twins should have scans every 2 weeks after 16 weeks to monitor for twin-to-twin transfusion syndrome and other complications.