title: Reproductive patterns among twins--Swedish register study of men and women born 1973-1983.
abstract: BACKGROUND
  During the last decades there has been a steady increase of twin births. A combination of improved medical treatment of preterm and small-for-gestational age children has contributed to a higher number of surviving twins. Prematurity is known to affect reproduction in a negative way. Few studies have focused on the potential effect twinning may have on future reproduction. Thus, the aim of this study was to investigate the effect of being born a twin compared to being born a singleton have on future reproduction.
  
  
  METHODS
  In a national population-based register study, all individuals born between 1973-1983 who were alive and living in Sweden at 13 years of age (n = 1 016 908) constituted the sample. Data on each study subject's own birth as well as the birth of their first offspring, and parental socio-demographic factors were collected from Swedish population based registers. Hazard ratios and corresponding 95% CI was calculated using Cox proportional hazards model.
  
  
  RESULTS
  Twins, both men and women, had a reduced likelihood of reproducing compared to singletons (women: HR = 0.89, 95% CI = 0.86-0.93; men: HR = 0.92, 95% CI = 0.87-0.97). This difference in birth rates can only partly be explained by diverging birth characteristics. Amongst men and women born very preterm, twins had an increased likelihood of reproducing compared to singletons (women: HR = 1.25, 95% CI = 1.02-1.62; men: HR = 1.34, 95% CI = 1.01-1.78).
  
  
  CONCLUSIONS
  Twins have lower reproduction rates compared to singletons, which only to a certain degree can be explained by diverging birth characteristics.
authors: Division of Obstetrics and Gynaecology, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University, Department of Obstetrics and Gynaecology in Linköping, County Council of Östergötland, Linköping, Sweden.
authors: Marie
authors: M
authors: Bladh
authors: Ann
authors: A
authors: Josefsson
authors: John
authors: J
authors: Carstensen
authors: Orvar
authors: O
authors: Finnström
authors: Gunilla
authors: G
authors: Sydsjö
mesh_terms: D000328:Adult; D001724:Birth Weight; D005260:Female; D005865:Gestational Age; D006801:Humans; D008297:Male; D011247:Pregnancy; D047928:Premature Birth / Q000453:epidemiology*; D016016:Proportional Hazards Models; D012042:Registries; D012098:Reproduction; D013548:Sweden; D014427:Twins / Q000706:statistics & numerical data*