Twins, DizygoticTwins, MonozygoticDiseases in TwinsTwinsMaternal Serum Screening TestsTwins, ConjoinedTwin Studies as TopicPregnancy, TwinPregnancy, MultipleTripletsHemizygoteChorionRh-Hr Blood-Group SystemEnvironmentPregnancyRegistriesZygoteBirth WeightReproductive Techniques, AssistedInfant, NewbornTwinning, MonozygoticGene-Environment InteractionGenetics, BehavioralGenotypeInheritance PatternsHomozygoteFetofetal TransfusionDenmarkHeredityQuantitative Trait, HeritableGenetic Predisposition to DiseaseModels, GeneticPhenotypeChimerismSwedenSocial Environment