2Rand L,Norwitz ER.Current controversies in cervical cerclage[J].Semin Perinatol,2003,27(1):73-85.
3Hassan SS,Romero R,Berry SM,et al.Patients with anultrasonographic cervical length《15mm have nearly a 50%risk of early spontaneous preterm delivery[J].Am J Obstet Gynecol,2000,182(6):1458-1467.
