The study is designed as an observational prospective cohort study. The primary outcome is complicated twin pregnancy (sIUGR, TTTS, IUFD, twin growth discordance). The main exposed factors are discordance of crown-rump length more than 20%, discordance of nuchal translucency more than 20%, either of twin with abnormal amniotic fluid (polyhydramnios or oligoamnios), either of twin with abnormal umbilical artery flow (elevated resistance index, absent or reverse diastolic flow), either of twin with abnormal middle cerebral artery flow (more than 1.5Mom), either of twin with abnormal ductus venous flow (absent or reverse a-wave). Meanwhile, some demographic data are to be collected and confounding factors are to be analyzed.