Twins, DizygoticTwins, MonozygoticDiseases in TwinsTwinsTwin Studies as TopicTwins, ConjoinedEnvironmentGenetics, BehavioralGene-Environment InteractionPregnancyPregnancy, TwinRegistriesQuantitative Trait, HeritableDenmarkGenetic Predisposition to DiseasePhenotypeSiblingsInfant, NewbornModels, GeneticSwedenTripletsBirth WeightPregnancy, Multiple