Women with twin pregnancy who are examined in the department are enrolled in this study. Ultrasound will be done in 11-13+6 weeks, 16-18 weeks, 22-24 weeks, 30-32 weeks and 37-39 weeks, and perinatal outcomes are to be collected, including the diagnosis of complicated twin pregnancy, when it happens, ultrasound indices, delivery mode, delivery time, birth weight, neonatal outcomes.