Bed RestPremature BirthInfant, PrematureInfant, NewbornPregnancyObstetric Labor, PrematureBirth WeightInfant, Low Birth WeightHead-Down TiltWeightlessness SimulationGestational AgePregnancy OutcomeWeightlessness CountermeasuresCongenital AbnormalitiesRestAbortion, VeterinaryInfant, Premature, DiseasesPregnancy ComplicationsCardiovascular DeconditioningBronchopulmonary DysplasiaFetal DeathSpace FlightInfant MortalityBedsAbortion, SpontaneousInfant, Small for Gestational AgePregnancy Complications, InfectiousRespiratory Distress Syndrome, NewbornWeightlessnessAnimals, NewbornRisk FactorsBirth RateOrthostatic IntoleranceLower Body Negative PressureRetrospective StudiesBirth OrderBirth CertificatesBed OccupancyTime FactorsSupine PositionMuscular AtrophyMuscle, SkeletalPlasma VolumeEarly AmbulationExerciseHypotension, OrthostaticHypovolemiaVascular CapacitanceAmino Acids, EssentialBirth IntervalsBirth InjuriesImmobilization