Aim: The aim of this study is to find out how common miscarriages are among women who have delivered a child.
Methods: The numbers of deliveries and miscarriages were extracted from the Swedish Medical Birth Register between 1983 and 2003. Linear regression was performed in order to investigate whether the increasing mean age of mothers or differences in pregnancy identification methods could explain the increased frequency of miscarriage.
Results: The reported number of miscarriages increased each year during the 21-year period, with a marked increase between 1991 and 1993 and only a slight increase during the final 10 years. For primiparous women, the frequency of reported miscarriages per delivery increased from 8.6% in 1983 to 13.9% in 2003. The corresponding figures for 2-parous women showed an increase from 14.5% to 21.3% respectively. Women aged 30-34 years had an odds ratio of 1.43 (95% CI 1.40-1.45) to suffer spontaneous abortion compared to the age group 25-29 years. Linear regression showed that an increase in mean age at delivery could only partly explain the increase in the frequency of reported miscarriages. A possible explanation could be differences in methods of identifying early pregnancy.
Conclusion: Of all women who deliver a child, nearly 20% have experienced previous miscarriage. The increased mean age of women could only explain a small portion of the seen increase in miscarriage. The marked increase from 1991 to 1993 is interesting. Possible reasons for the increase are discussed.