Cancer of the breast is common in developed countries where the lifetime risk ranges from 1 in 12 to 1 in 20 women. Among women living in developing countries the risk is lower but appears to be increasing. Risk factors for breast cancer include high socio-economic status, early menarche, late first birth, late menopause, and a family history of breast cancer.
Currently, in countries where induced abortion is permitted by law, which have total fertility rates of about 2 or less and high prevalence rates of contraceptive use, the annual rate of induced abortion is 1 to 2 abortions per 100 women of reproductive age. In these countries on average, almost every second woman has an abortion during her life. In countries with similar fertility rates but much lower prevalence of contraceptive use, the annual rate of induced abortion is higher and can be estimated to be up to 10 or more per 100 women annually. In countries where abortion is permitted by the law, the large majority of abortions (typically > 90 %) take place before the end of the 12th week of pregnancy.
Most of the information on whether induced abortion modifies the risk of breast cancer among women comes from epidemiological studies, which are either case-control studies, or historical cohort studies. For information on abortion, all published case-control studies have relied on interviews of cases and controls with the inherent problem of recall bias. This bias occurs because women with breast cancer (cases) tend to truthfully report induced abortion while controls, who often are healthy women, have no "incentive" to provide information about personal and sensitive matters such as induced abortion. Such bias can produce elevated relative risk estimates in case-control studies. As a result, the outcome of such studies has been inconsistent, with some having indicated a small increase in risk, while others have not.
Historical cohort studies, on the other hand, are more methodologically sound. Two major studies have been carried out using this methodology, and neither found an increased risk of breast cancer associated with first trimester abortion.
Therefore, results from epidemiological studies are reassuring in that they show no consistent effect of first trimester induced abortion upon a woman's risk of breast cancer later in life.