Reproductive patterns among twins - a Swedish register study of men and women born 1973-1983

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.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).Twins have lower reproduction rates compared to singletons, which only to a certain degree can be explained by diverging birth characteristics.Worldwide there has been a steady increase of twin births in Sweden the rate rose from 8.2 births/1000 women in 1973 to 16.3 births/1000 women in 2003 [1-3]. Lately this increase can be attributed to IVF-treatment and hormone stimulation but also to the increasing maternal age [2,4]. This in combination with the improvement of medical treatment of preterm and/or small for gestational age (SGA) children has contributed to a higher number of surviving twins. In many cases twins are born preterm and with a low birthweight [5].Ekholm et al. [6] showed that women born with very low birthweight had a reduced likelihood of reproducing. On the other hand, women born SGA had an increased likelihood of reproducing. A later study on partially the same but now enlarged material confirmed the decreased likelihood of reproducing for both men and women born preterm, but not the increased likelihood for women born SGA who seemed to start having children at