title: An international contrast of rates of placental abruption: an age-period-cohort analysis.
abstract: BACKGROUND
  Although rare, placental abruption is implicated in disproportionately high rates of perinatal morbidity and mortality. Understanding geographic and temporal variations may provide insights into possible amenable factors of abruption. We examined abruption frequencies by maternal age, delivery year, and maternal birth cohorts over three decades across seven countries.
  
  
  METHODS
  Women that delivered in the US (n = 863,879; 1979-10), Canada (4 provinces, n = 5,407,463; 1982-11), Sweden (n = 3,266,742; 1978-10), Denmark (n = 1,773,895; 1978-08), Norway (n = 1,780,271, 1978-09), Finland (n = 1,411,867; 1987-10), and Spain (n = 6,151,508; 1999-12) were analyzed. Abruption diagnosis was based on ICD coding. Rates were modeled using Poisson regression within the framework of an age-period-cohort analysis, and multi-level models to examine the contribution of smoking in four countries.
  
  
  RESULTS
  Abruption rates varied across the seven countries (3-10 per 1000), Maternal age showed a consistent J-shaped pattern with increased rates at the extremes of the age distribution. In comparison to births in 2000, births after 2000 in European countries had lower abruption rates; in the US there was an increase in rate up to 2000 and a plateau thereafter. No birth cohort effects were evident. Changes in smoking prevalence partially explained the period effect in the US (P = 0.01) and Sweden (P<0.01).
  
  
  CONCLUSIONS
  There is a strong maternal age effect on abruption. While the abruption rate has plateaued since 2000 in the US, all other countries show declining rates. These findings suggest considerable variation in abruption frequencies across countries; differences in the distribution of risk factors, especially smoking, may help guide policy to reduce abruption rates.
authors.affiliation: Department of Obstetrics and Gynecology, College of Physicians and Surgeons, Columbia University, New York, New York, United States of America; Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, New York, United States of America.
authors.forename: Cande V
authors.initials: CV
authors.lastname: Ananth
authors.affiliation: Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, New York, United States of America.
authors.forename: Katherine M
authors.initials: KM
authors.lastname: Keyes
authors.affiliation: Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, New York, United States of America.
authors.forename: Ava
authors.initials: A
authors.lastname: Hamilton
authors.affiliation: THL National Institute for Health and Welfare, Helsinki, Finland and NHV Nordic School of Public Health, Gothenburg, Sweden.
authors.forename: Mika
authors.initials: M
authors.lastname: Gissler
authors.affiliation: Section for Epidemiology, Department of Public Health, Aarhus University, Aarhus, Denmark.
authors.forename: Chunsen
authors.initials: C
authors.lastname: Wu
authors.affiliation: Health Surveillance and Epidemiology Division, Centre for Chronic Disease Prevention, Public Health Agency of Canada, Ottawa, Canada.
authors.forename: Shiliang
authors.initials: S
authors.lastname: Liu
authors.affiliation: Department of Epidemiology, School of Public Health, Harvard University, Boston, Massachusetts, United States of America.
authors.forename: Miguel Angel
authors.initials: MA
authors.lastname: Luque-Fernandez
authors.affiliation: Department of Global Public Health and Primary Care, University of Bergen, Bergen, Norway; Medical Birth Registry of Norway, Norwegian Institute of Public Health, Bergen, Norway.
authors.forename: Rolv
authors.initials: R
authors.lastname: Skjærven
authors.affiliation: Department of Epidemiology, School of Public Health, Harvard University, Boston, Massachusetts, United States of America.
authors.forename: Michelle A
authors.initials: MA
authors.lastname: Williams
authors.affiliation: Department of Obstetrics and Gynecology, University Central Hospital, Helsinki, Finland.
authors.forename: Minna
authors.initials: M
authors.lastname: Tikkanen
authors.affiliation: Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
mesh_terms: D000037:Abruptio Placentae / Q000453:epidemiology*; D017677:Age Distribution; D002170:Canada / Q000453:epidemiology; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D005260:Female; D005387:Finland / Q000453:epidemiology; D006801:Humans; D008423:Maternal Age; D009664:Norway / Q000453:epidemiology; D011247:Pregnancy; D015995:Prevalence; D012307:Risk Factors; D012907:Smoking; D013030:Spain / Q000453:epidemiology; D013548:Sweden; D014481:United States / Q000453:epidemiology