The topic of abortion generates passionate viewpoints in many people, which are often linked to personal religious or political beliefs, rather than reliance on scientific evidence.
A randomised study is the most reliable way to know if something causes cancer. A randomised study takes a large number of people, and exposes only half of them to the thing that could cause cancer (the possible carcinogen). Study participants are randomly allocated to the exposed or non-exposed group. This random allocation helps to ensure that the two groups are similar in terms of other cancer risk factors. Using this kind of research study, the researchers can be confident that any difference in the number of people in each group who get cancer is likely as a result of being exposed to the possible carcinogen. However, this type of study is not always practical or ethical, which is true for the effects of abortion on cancer risk – it would not be possible or ethical to randomly allocate pregnant women to either undergo an abortion or not. Indeed, it would not be possible or ethical to randomly allocate any group of people to be exposed to a potential carcinogen or not – the risks to research participants of this sort of study would be deemed to be unacceptably high.
For areas where it is not practical or ethical to use a randomised study, such as exposing people to a possible carcinogen, observational methods, such as case-control or cohort studies are used. In a case-control study, a group of people who already have a disease are compared to a group of people who do not have the disease, to identify differences in their past exposures to risk factors. Studies such as these, which have ‘retrospective’ designs (that is, which are which are designed to ‘look back’) have inherent flaws, in that people can find it hard to remember what they did in the past, which means that the study could miss an important link between a risk factor and the disease. On the other hand, retrospective research designs can also result in a study identifying a link that may not really exist due to recall bias. Recall bias refers to the fact that people who have the disease being studied are more likely to remember things that healthy people do not; and are more likely to report to researchers things that they would otherwise feel were too personal or embarrassing.
In a cohort study, researchers gather baseline information about a group of people who don’t have the disease being studied, and then watch these people over time, often asking questions about the possible risk factor or exposure of interest. After a long period of time (the ‘follow up period’), the researchers look to see who acquired the disease and who did not. ‘Prospective’ studies such as these (that is, studies which ‘look forward’) are less prone to recall bias, and the conclusions that can be drawn from them are considered stronger than those from case-control studies.
Results of studies looking at the potential link between abortion and breast cancer risk differ depending on the type of study. Cohort studies have not found an increased risk, while some case-control studies have found an increased risk. The myth that abortion causes breast cancer has resurfaced due to a recently published meta-analysis (a method which contrasts and combines results from multiple studies, in the hope of identifying patterns among the study results) that concluded that there was an association between abortion and breast cancer in Chinese women. The meta-analysis included 36 studies – 34 of which were case-control studies, while the remaining two were cohort studies. The two cohort studies included in this meta-analysis did not find an increased risk of breast cancer in women who had a history of abortion. In other words, in this meta-analysis, only the studies with the weaker research design suggested a link between abortion and breast cancer. Both of the studies with the stronger design did not find evidence of such a link.
In summary, research evidence does not suggest that abortion increases the risk of breast cancer in women. Women who want to reduce their risk of breast cancer would be well advised to concentrate on risk factors that evidence has clearly shown increase the risk of breast cancer.