Bed RestPremature BirthInfant, PrematureInfant, NewbornPregnancyObstetric Labor, PrematureBirth WeightHead-Down TiltInfant, Low Birth WeightWeightlessness SimulationGestational AgeWeightlessness CountermeasuresPregnancy OutcomeRestCongenital AbnormalitiesAbortion, VeterinaryInfant, Premature, DiseasesPregnancy ComplicationsCardiovascular DeconditioningBronchopulmonary DysplasiaSpace FlightFetal DeathBedsAbortion, SpontaneousInfant MortalityInfant, Small for Gestational AgePregnancy Complications, InfectiousWeightlessnessRespiratory Distress Syndrome, NewbornAnimals, NewbornOrthostatic IntoleranceRisk FactorsBirth RateLower Body Negative PressureRetrospective StudiesBirth OrderBirth CertificatesBed OccupancySupine PositionMuscular AtrophyMuscle, SkeletalPlasma VolumeEarly AmbulationExerciseTime FactorsHypotension, OrthostaticHypovolemiaVascular CapacitanceAmino Acids, EssentialBirth IntervalsImmobilization