title: Reference population for international comparisons and time trend surveillance of preterm delivery proportions in three countries.
abstract: BACKGROUND
  International comparison and time trend surveillance of preterm delivery rates is complex. New techniques that could facilitate interpretation of such rates are needed.
  
  
  METHODS
  We studied all live births and stillbirths (>or= 28 weeks gestation) registered in the medical birth registers in Sweden, Denmark and Norway from 1995 through 2004. Gestational age was determined by best estimate. A reference population of pregnant women was designed using the following criteria: 1) maternal age 20-35, 2) primiparity, 3) spontaneously conceived pregnancy, 4) singleton pregnancy and 5) mother born in the respective country. National preterm delivery rate, preterm delivery rate in the reference population and rate of spontaneous preterm delivery in the reference population were calculated for each country.
  
  
  RESULTS
  The total national preterm delivery rate (< 37 completed gestational weeks), increased in both Denmark (5.3% to 6.1%, p < 0.001) and Norway (6.0% to 6.4%, p = 0.006), but remained unchanged in Sweden, during 1995-2004. In Denmark, the preterm delivery rate in the reference population (5.3% to 6.3%, p < 0.001) and the spontaneous preterm delivery rate in the reference population (4.4% to 6.8%, p < 0.001) increased significantly. No similar increase was evident in Norway. In Sweden, rates in the reference population remained stable.
  
  
  CONCLUSIONS
  Reference populations can facilitate overview and thereby explanations for changing preterm delivery rates. The model also permits comparisons over time. This model may in its simplicity prove to be a valuable supplement to assessments of national preterm delivery rates for public health surveillance.
authors.affiliation: Department of Obstetrics and Gynecology, Telemark Hospital, Skien, Norway. nhmorken@online.no
authors.forename: Nils-Halvdan
authors.initials: NH
authors.lastname: Morken
authors.forename: Ida
authors.initials: I
authors.lastname: Vogel
authors.forename: Karin
authors.initials: K
authors.lastname: Kallen
authors.forename: Rolv
authors.initials: R
authors.lastname: Skjaerven
authors.forename: Jens
authors.initials: J
authors.lastname: Langhoff-Roos
authors.forename: Ulrik Schiøler
authors.initials: US
authors.lastname: Kesmodel
authors.forename: Bo
authors.initials: B
authors.lastname: Jacobsson
mesh_terms: D000328:Adult; D001719:Birth Certificates; D003431:Cross-Cultural Comparison; D003718:Denmark / Q000453:epidemiology; D005260:Female; D005865:Gestational Age; D006801:Humans; D015994:Incidence; D007231:Infant, Newborn; D050498:Live Birth / Q000453:epidemiology; D009664:Norway / Q000453:epidemiology; D010298:Parity; D011159:Population Surveillance / Q000379:methods*; D011247:Pregnancy; D047928:Premature Birth / Q000453:epidemiology*; D012016:Reference Values; D050497:Stillbirth / Q000453:epidemiology; D013548:Sweden / Q000453:epidemiology