Vaginal Progesterone in Twins With Short Cervix

Inclusion Criteria: * Gestational age between 20w0d and 24w0d * Twin pregnancy, diamniotic-dichorionic, both twins living * Cervical length 10 mm to 25 mm discovered by a screening transvaginal ultrasound exam Exclusion Criteria: * Mother less than 18 years of age * Uterine contractions of 40 seconds duration or more, 10 or more per hour * Rupture of fetal membranes (leakage of amniotic fluid one or both sacs) * Ongoing vaginal bleeding * Any condition likely to cause serious neonatal morbidity independent of gestational age, including: 1. fetal malformation likely to require surgery 2. fetal malformation involving vital organs 3. fetal viral infection 4. hydrops fetalis 5. discordance in estimated fetal weight more than 10% 6. velamentous insertion of umbilical cord of either twin 7. placenta previa of either placenta * Any contraindication to continuing the pregnancy * Cervical cerclage in place or planned * Any contraindication to vaginal micronized progesterone, including: 1. Known sensitivity to progesterone or any of the other ingredients 2. Liver dysfunction or disease 3. Known or suspected malignancy of breast or genital organs 4. Active thromboembolic disorder, or history of hormone-associated thromboembolic disorder