This study is for compare the efficacy of two different regimens(vaginal vs intramuscular) of progesterone therapy in prevention of preterm birth in high risk pregnant women.

The objectives of this study is 1) To compare the efficacy of two different regimens of progesterone therapy in preventing preterm birth less than 37 weeks of gestation. 2) To compare the cost-benefit, adverse effects and patient preference of two different regimens of progesterone therapy.

Design: Multi-center, randomized, open-label trial

Inclusion Criteria:

• With a history of prior spontaneous preterm birth or short cervix length
• Spontaneous preterm birth: preterm birth <37 weeks of gestation due to spontaneous preterm labor or preterm premature rupture of membranes (PPROM)
• Short Cervix length : cervix length <25 mm as measured by transvaginal ultrasound at 16-24 weeks of gestation

Exclusion Criteria:

• Multiple gestations
• Major congenital anomalies
• Elective prophylactic cervical cerclage <16 weeks of gestation during current pregnancy
• Previous intentional preterm birth due to maternal or fetal indications, such as preeclampsia and fetal growth restriction
• Diabetes, hypertension, other severe medical diseases