Objective: We wanted to study to what extent induced abortion rates influence the age-dependent estimates of miscarriage risk.
Design: Register-based study.
Setting: Linkage between hospital records and the Norwegian Population Registry.
Population: Included were all women 15-49 years of age in Oslo, Norway, who were pregnant during the three-year study period, 27,932 women.
Methods: We estimated proportions with miscarriage according to maternal age with and without inclusion of women with induced abortion in the denominator.
Main outcome measure: Proportions with miscarriage in each age group and odds ratios of miscarriage, with age group 25-29 years as the reference.
Results: When only women with pregnancies intended to term were included in the denominator, 9.4% of the women were estimated to have miscarriage as the only reproductive event during the three-year period and 6.8% had both miscarriage and delivery. The corresponding figures when also women with induced abortions were included in the denominator, were 7.0 and 5.0%. When taking induced abortion into account, the reduction in estimated proportion with miscarriage during the study period was greatest in the age groups with the highest induced abortion rates, women 15-19 years old (from 22.8 to 6.4%) and women 45-49 years old (from 57.9 to 41.0%).
Conclusions: Estimates of age-dependent risk of miscarriage may be strongly influenced by induced abortion rates.