title: Physical activity and the risk of preterm birth: a systematic review and meta-analysis of epidemiological studies.
abstract: BACKGROUND
  Physical activity has been inconsistently associated with risk of preterm birth, and the strength of the association and the shape of the dose-response relationship needs clarification.
  
  
  OBJECTIVE
  To conduct a systematic review and dose-response meta-analysis to clarify the association between physical activity and risk of preterm birth.
  
  
  METHODS
  PubMed, Embase and Ovid databases were searched for relevant studies up to 9 February 2017.
  
  
  METHODS
  Studies with a prospective cohort, case-cohort, nested case-control or randomized study design were included.
  
  
  METHODS
  Data were extracted by one reviewer and checked for accuracy by a second reviewer. Summary relative risks (RRs) were estimated using a random effects model.
  
  
  RESULTS
  Forty-one studies (43 publications) including 20 randomized trials and 21 cohort studies were included. The summary RR for high versus low activity was 0.87 [95% confidence interval (CI): 0.70-1.06, I2= 17%, n = 5] for physical activity before pregnancy, and it was 0.86 (95% CI: 0.78-0.95, I2= 0%, n = 30) for early pregnancy physical activity. The summary RR for a 3 hours per week increment in leisure-time activity was 0.90 (95% CI: 0.85-0.95, I2= 0%, n = 5). There was evidence of a nonlinear association between physical activity and preterm birth, Pnonlinearity< 0.0001, with the lowest risk observed at 2-4 hours per week of activity.
  
  
  CONCLUSIONS
  This meta-analysis suggests that higher leisure-time activity is associated with reduced risk of preterm birth. Further randomized controlled trials with sufficient frequency and duration of activity to reduce the risk and with larger sample sizes are needed to conclusively demonstrate an association.
  
  
  CONCLUSIONS
  Physically active compared with inactive women have an 10-14% reduction in the risk of preterm birth.
authors.affiliation: Department of Public Health and General Practice, Faculty of Medicine, Norwegian University of Science and Technology, Trondheim, Norway.|Department of Epidemiology and Biostatistics, School of Public Health, Imperial College London, London, UK.|Bjørknes University College, Oslo, Norway.
authors.forename: D
authors.initials: D
authors.lastname: Aune
authors.affiliation: Department of Epidemiology and Biostatistics, School of Public Health, Imperial College London, London, UK.
authors.forename: S
authors.initials: S
authors.lastname: Schlesinger
authors.affiliation: Section of Obstetrics, Division of Obstetrics and Gynaecology, Rikshospitalet, Oslo University Hospital, Oslo, Norway.
authors.forename: T
authors.initials: T
authors.lastname: Henriksen
authors.affiliation: Department of Pediatric Research, Rikshospitalet, Oslo University Hospital, University of Oslo, Oslo, Norway.
authors.forename: O D
authors.initials: OD
authors.lastname: Saugstad
authors.affiliation: Department of Preventive Cardiology, Oslo University Hospital, University of Oslo, Oslo, Norway.
authors.forename: S
authors.initials: S
authors.lastname: Tonstad
mesh_terms: D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D011247:Pregnancy; D047928:Premature Birth / Q000453:epidemiology / Q000209:etiology*; D011295:Prenatal Care / Q000379:methods; D039401:Prenatal Nutritional Physiological Phenomena; D012307:Risk Factors
keywords: Meta-analysis; physical activity; preterm birth