Bed RestPremature BirthTwinsDiseases in TwinsInfant, PrematureTwins, MonozygoticInfant, NewbornPregnancyObstetric Labor, PrematureTwins, ConjoinedTwins, DizygoticBirth WeightHead-Down TiltInfant, Low Birth WeightWeightlessness SimulationGestational AgePregnancy OutcomeWeightlessness CountermeasuresCongenital AbnormalitiesRestAbortion, VeterinaryPregnancy ComplicationsInfant, Premature, DiseasesCardiovascular DeconditioningBronchopulmonary DysplasiaFetal DeathSpace FlightAbortion, SpontaneousBedsTwin Studies as TopicInfant MortalityInfant, Small for Gestational AgePregnancy Complications, InfectiousPregnancy, TwinWeightlessnessRespiratory Distress Syndrome, NewbornAnimals, NewbornRisk FactorsBirth RateOrthostatic IntoleranceLower Body Negative PressureRetrospective StudiesBirth OrderPregnancy, MultipleBirth CertificatesBed OccupancySupine PositionMuscle, SkeletalMuscular AtrophyTime FactorsExercisePlasma VolumeEarly AmbulationHypotension, OrthostaticHypovolemiaBirth IntervalsVascular CapacitanceAmino Acids, EssentialFetofetal TransfusionImmobilizationTripletsMaternal Age