Recent years, women with infertility have become more and more and thus assisted reproductive technology has been applied in a broad range. And the quantities of twin pregnancies are larger and larger. However, complicated twin pregnancy subsequently increased including selective intrauterine growth restriction (sIUGR), twin growth discordance, twin transfusion syndrome (TTTS) and intrauterine fetal death (IUFD). The occurrence of complicated twin pregnancy has been the leading cause of morbidity and mortality in twin and mother. Meanwhile, twin pregnancy, especially monochorionic type has suffered from higher rate of chromosomal anomalies and structural anomalies. Therefore, it is important that more attention should be paid to the prediction, clinical evaluation and the occurrence of chromosomal anomalies and structural anomalies in complicated twin pregnancy.

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.

The goals of this study are as below: (1) primary goal: the prediction of complicated twin pregnancy by using prenatal ultrasound; (2) primary goal: the clinical evaluation of perinatal outcomes in twin pregnancy by using prenatal ultrasound; (3) secondary goal: the analysis of the correlation between prenatal ultrasound and fetal anomalies.

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.

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.

When there are indications for the detection of fetal chromosome, women are probably advised to have this examination, such as non-invasive DNA, amniocentesis, umbilical cord screening, as appropriate, and written informed consent will be achieved from every patient.

Inclusion Criteria:

• women with twin pregnancy starting at 11-14 weeks of gestation.

Exclusion Criteria:

• women with triplet pregnancy;
• complicating with acute internal or surgical diseases.

pregnant women who visit the First Affiliated Hospital of Xian Jiaotong University.