Multiple Birth OffspringTwins, MonozygoticDiseases in TwinsPregnancy, MultipleTwinsTwins, DizygoticTwinning, MonozygoticPregnancyTwins, ConjoinedTwin Studies as TopicTripletsPregnancy, TwinDermatoglyphicsEnvironmentQuadrupletsQuintupletsFetofetal TransfusionPhenotypePregnancy Reduction, MultifetalGenetic Predisposition to DiseaseGene-Environment InteractionInfant, NewbornRegistriesInheritance PatternsEmbryo TransferGenetics, BehavioralHeredityBirth WeightMosaicismSocial EnvironmentDenmarkModels, GeneticAbnormalities, MultipleTurner SyndromeMicromanipulationChorionBlood Group AntigensSingle Embryo TransferVietnam Conflict