The risk of abnormal chromosome and structure is much higher in twins than in singletons, and traditional early pregnancy screening strategy for single pregnancy is not suitable for twins. Based on our management experience of fetal medicine at twin pregnancy, and multi-center cooperation, the study will carry out the following clinical studies:

1. to explore a suitable, early, noninvasive and accurate prenatal screening strategy for twin pregnancy.
2. fetal chromosomal abnormalities

Inclusion Criteria:

• ultrasound diagnosis as twin pregnancy
• at 11-14 gestational weeks

Exclusion Criteria:

• Multiple pregnancies (three or more)
• Maternal history of chromosome aneuploidy abnormality or tumor

The Individual Participant Data sharing plan will be discussed with the other collaborators