Vaginal Compared With Intramuscular Progesterone for Prevention of Preterm Birth in High Risk Pregnant Women

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