Inclusion Criteria:  twins with cervical length ≤ 25 mm by vaginal ultrasound at 14-32 weeks no contractions, abdominal pain and vaginal bleeding No complicated twin complications occurred during pregnancy  Exclusion Criteria:  Patients undergoing selective cervical ligation before 14 weeks of gestation history of liver problems or cholestasis during pregnancy Abnormal liver enzymes Abnormal renal function Local allergy to trace natural progesterone Recurrent vaginal bleeding Recurrent vaginal infection Ultrasound diagnosis of fetal abnormality Intrauterine treatment for fetal abnormalities