Twins, DizygoticTwins, MonozygoticDiseases in TwinsTwinsTwins, ConjoinedTwin Studies as TopicPregnancy, TwinEnvironmentTripletsPregnancy, MultipleTwinning, MonozygoticPregnancyGene-Environment InteractionGenetics, BehavioralRegistriesInheritance PatternsMaternal Serum Screening TestsFetofetal TransfusionDenmarkHeredityQuantitative Trait, HeritableBirth WeightGenetic Predisposition to DiseaseModels, GeneticInfant, NewbornPhenotypeChimerismChorionSwedenSocial EnvironmentSiblingsFinlandCohort StudiesRisk FactorsMaternal Age