Verify the Safety and Effectiveness of the Cerclage Pessary in Prevention and Treatment of High-risk Preterm Pregnancy

progeaterone:Utrogestan is one of the normal drugs in China to prevent spontaneous preterm birth, the Utrogestan used in this study is from Laboratories Besins International.

pessary: The cervical pessary is a vaginal device (silicone ring) that is used to treat pregnant women for preventing spontaneous preterm birth.