title: Increasing twinning rates in Norway, 1967-2004: the influence of maternal age and assisted reproductive technology (ART).
abstract: BACKGROUND
  The incidence of twin pregnancies is increasing in industrialized countries, including Norway. In the present nationwide study from Norway, we aimed at evaluating the effect of assisted reproductive technologies and delayed childbearing age on this epidemic rise.
  
  
  METHODS
  Altogether 2.19 million pregnancies, including 27,849 twin pairs and higher-order multiples, were included in this population-based study with data from the Medical Birth Registry of Norway covering the years 1967-2004. The study period was divided into two, 1967-87 and 1988-2004, respectively. For the last time period, pregnancies from assisted reproductive technologies were available through a separate registration, and these data were linked with the Medical Birth Registry of Norway birth record. Additionally, we compared the twinning rates from natural conception in the two time periods in relation to maternal age. Logistic regression was used for adjusted odds ratio estimation.
  
  
  RESULTS
  During 1967-1987, the twinning rate remained constant at 1.0% of the total number of births in Norway. Thereafter, the total twinning rate increased from 1.1% in 1988 to 1.9% in 2004. After excluding pregnancies from assisted reproductive technologies, the increase was from 1.1% to 1.6% in this period. There was a significantly higher twinning rate in all age groups in the time period 1988-2004 compared with 1967-1987. In naturally conceived pregnancies, OR for twins were 1.11 (95% CI 1.05-1.17) in the age group 20-24 years, 1.25 (95% CI 1.19-1.31) in the age group 30-34, and 1.36 (95% CI 1.11-1.67) in the age group 40-44 comparing the two time periods.
  
  
  CONCLUSIONS
  The multiple pregnancy rate increased by 50% in Norway during the time period 1988-2004, even when twin pregnancies from assisted reproduction were excluded. Maternal age and assisted reproductive technologies cannot alone satisfactorily explain the rise in the twinning rate in Norway.
authors.affiliation: Department of Obstetrics and Gynaecology, Haukeland University Hospital, Bergen, Norway. anne.tandberg@helse-bergen.no
authors.forename: Anne
authors.initials: A
authors.lastname: Tandberg
authors.forename: Tone
authors.initials: T
authors.lastname: Bjørge
authors.forename: Per E
authors.initials: PE
authors.lastname: Børdahl
authors.forename: Rolv
authors.initials: R
authors.lastname: Skjaerven
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D006801:Humans; D016015:Logistic Models; D008423:Maternal Age; D009664:Norway / Q000453:epidemiology; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000706:statistics & numerical data*; D012042:Registries; D027724:Reproductive Techniques, Assisted; D012189:Retrospective Studies; D014427:Twins