Bed RestPremature BirthTripletsInfant, PrematureInfant, NewbornPregnancyObstetric Labor, PrematureBirth WeightHead-Down TiltInfant, Low Birth WeightWeightlessness SimulationGestational AgePregnancy OutcomeWeightlessness CountermeasuresCongenital AbnormalitiesRestAbortion, VeterinaryPregnancy ComplicationsInfant, Premature, DiseasesCardiovascular DeconditioningBronchopulmonary DysplasiaFetal DeathSpace FlightAbortion, SpontaneousPregnancy, TripletBedsInfant MortalityInfant, Small for Gestational AgePregnancy Complications, InfectiousWeightlessnessRespiratory Distress Syndrome, NewbornAnimals, NewbornBirth RateRisk FactorsOrthostatic IntoleranceTrinucleotide RepeatsLower Body Negative PressureRetrospective StudiesPregnancy, MultipleBirth OrderBirth CertificatesBed OccupancyTime FactorsMuscle, SkeletalSupine PositionMuscular AtrophyMultiple Birth OffspringPregnancy Reduction, MultifetalPlasma VolumeEarly AmbulationExerciseHypotension, OrthostaticTrinucleotide Repeat ExpansionHypovolemiaVascular CapacitanceAmino Acids, EssentialBirth IntervalsQuadrupletsImmobilizationMaternal Age