Preventing Preterm Birth With a Pessary

Inclusion Criteria: * Singleton pregnancies in patients with a history of spontaneous preterm birth (previous delivery between 17 0/7 weeks and 36 6/7 weeks). * Women ages 18 to 45 years of age Exclusion Criteria: * Major fetal abnormalities (defined as those that are lethal or require prenatal or postnatal surgery), fetal death, or fetal growth restriction diagnosed before randomization. * Presence of prophylactic cervical cerclage * Significant maternal-fetal complications (treated chronic hypertension, insulin dependant diabetes mellitus, red cell isoimmunization) * Painful regular uterine contractions, or ruptured membranes * Visual cervical dilation of 2cm or greater and visible membranes. * Patients with a pregnancy dated by an ultrasound after 20 weeks gestation.