So far, prenatal ultrasound has been acknowledged as the best method for prenatal diagnosis and evaluation in twin pregnancy. By means of the application of prenatal ultrasound, the detection of fetal chromosome, and secondary correlation analysis, the investigators try to build prenatal ultrasound monitoring system of twin pregnancy and provide evidences for the choice of intervention and delivery time, in order to decrease the morbidity and mortality of twin and improve perinatal short and long outcomes.