TwinsDiseases in TwinsTwins, MonozygoticTwins, ConjoinedTwins, DizygoticTwin Studies as TopicPregnancy, TwinPregnancy, MultipleFetofetal TransfusionPregnancyTripletsEnvironmentPregnancy Reduction, MultifetalGestational AgeInfant, NewbornFetal DeathPregnancy OutcomeChorionGenetics, BehavioralUltrasonography, PrenatalBirth WeightGene-Environment InteractionTwinning, MonozygoticSocial EnvironmentRegistriesFetoscopyPhenotypeRisk FactorsQuadrupletsGenetic Predisposition to DiseaseDenmarkPolyhydramniosSwedenDermatoglyphicsReproductive Techniques, AssistedCohort StudiesModels, GeneticTime FactorsTwin StudyFertilization in VitroBirth Order