Premature BirthHydroxyprogesteronesTocolytic AgentsPregnancyObstetric Labor, PrematureInfant, PrematureInfant, NewbornRecurrenceGestational AgeBirth WeightInfant, Low Birth WeightPregnancy OutcomeFetal Membranes, Premature RuptureCerclage, CervicalInfant, Premature, DiseasesBirth RatePregnancy ComplicationsEstradiolProgestinsUterine Cervical IncompetenceInfant, Small for Gestational AgeCervical Length MeasurementChorioamnionitisInfant MortalityBirth CertificatesTerm BirthCervix UteriStillbirthMifepristoneProgesterone CongenersCorpus LuteumUterusPregnancy, High-RiskAdministration, IntravaginalPregnancy Trimester, SecondInfant, Extremely PrematureRisk FactorsPrenatal CareFetal DeathMaternal AgeEstrus