Multiple Birth OffspringPregnancy, MultipleTwinsDiseases in TwinsPregnancyTripletsTwins, MonozygoticBirth RateTwins, ConjoinedTwins, DizygoticQuadrupletsBirth WeightReproductive Techniques, AssistedMaternal AgeInfant, NewbornPremature BirthInfant, Low Birth WeightPregnancy OutcomeInfant MortalityBirth OrderFetal DeathGestational AgeEmbryo TransferTwin Studies as TopicFertilization in VitroCongenital AbnormalitiesPregnancy, TwinBirth CertificatesPregnancy Reduction, MultifetalPerinatal CareInfertilityDelivery, ObstetricLive BirthInfant, PrematurePregnancy ComplicationsRisk FactorsInfant, Premature, DiseasesVital StatisticsSingle Embryo TransferFertility Agents, FemaleCesarean SectionIntensive Care Units, NeonatalSperm Injections, IntracytoplasmicIntensive Care, NeonatalUnited StatesPrenatal CareOvulation InductionRegistriesParity