Could a simple silicone ring prevent preterm birth?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Singleton gestation. Twin gestation reduced to singleton either spontaneously or therapeutically, is not eligible unless the reduction occurred before 13 weeks 6 days project gestational age. Higher order multifetal gestations reduced to singletons are not eligible. * Gestational age at randomization between 16 weeks 0 days and 23 weeks 6 days based on clinical information and evaluation of the earliest ultrasound. * Cervical length on transvaginal examination of less than or equal to 20 mm by a study certified sonographer. There is no lower cervical length threshold. Exclusion Criteria: * Cervical dilation (internal os) 3 cm or greater on digital examination or evidence of prolapsed membranes beyond the external cervical os either at the time of the qualifying cervical ultrasound examination or at a cervical exam immediately before randomization. * Fetal anomaly or imminent fetal demise. This includes lethal anomalies, or anomalies that may lead to early delivery or increased risk of neonatal death e.g., gastroschisis, spina bifida, serious karyotypic abnormalities. An ultrasound examination from 14 weeks 0 days to 23 weeks 6 days by project Estimated Date of Confinement (EDC) must be performed prior to randomization to evaluate the fetus for anomalies. * Previous spontaneous preterm birth between 16 weeks 0 days and 36 weeks 6 days. This includes induction for pPROM in a prior pregnancy. * Planned treatment with intramuscular 17-α hydroxy-progesterone caproate. * Placenta previa, because of risk of bleeding and high potential for indicated preterm birth. A low lying placenta is acceptable. * Active vaginal bleeding greater than spotting at the time of randomization, because of potential exacerbation due to pessary placement. * Symptomatic, untreated vaginal or cervical infection because of potential exacerbation due to pessary placement. Patients may be treated and if subsequently asymptomatic, randomized. However, if it is more than 10 days since the cervical length measurement, a new cervical length measurement must be obtained. * Active, unhealed herpetic lesion on labia minora, vagina, or cervix due to the potential for significant patient discomfort or increasing genital tract viral spread. Once lesion(s) heal and the patient is asymptomatic, she may be randomized. History of herpes is not an exclusion. * Rupture of membranes due to likelihood of pregnancy loss and preterm delivery as well as the risk of ascending infection which could be increased with pessary placement. * More than six contractions per hour reported or documented prior to randomization. It is not necessary to place the patient on a tocodynamometer. * Known major Mullerian anomaly of the uterus (specifically bicornuate, unicornuate, or uterine septum not resected) due to increased risk of preterm delivery which is unlikely to be affected by progesterone. * Any fetal/maternal condition which would require invasive in-utero assessment or treatment, for example significant red cell antigen sensitization or neonatal alloimmune thrombocytopenia. * Major maternal medical illness associated with increased risk for adverse pregnancy outcome or indicated preterm birth (treated hypertension requiring more than one agent, treatment for diabetes prior to pregnancy, chronic renal insufficiency defined by creatinine \>1.4 mg/dL, carcinoma of the breast, conditions treated with chronic oral glucocorticoid therapy). Lupus, uncontrolled thyroid disease, and New York Heart Association(NYHA) stage II or greater cardiac disease are also excluded. Patients with seizure disorders, HIV, and other medical conditions not specifically associated with an increased risk of indicated preterm birth are not excluded. Prior cervical cone/LOOP/Loop Electrocautery Excision Procedure (LEEP) is not an exclusion criterion. * Planned cerclage or cerclage already in place since it would preclude placement of a pessary. * Planned indicated delivery prior to 37 weeks. * Allergy to silicone. * Participation in another interventional study that influences gestational age at delivery or neonatal morbidity or mortality. * Participation in this trial in a previous pregnancy. Patients who were screened in a previous pregnancy, but not randomized, may be included. * Prenatal care or delivery planned elsewhere unless the study visits can be made as scheduled and complete outcome information can be obtained.