New Jersey breast cancer surgeon, Angela Lanfranchi, M.D., F.A.C.S.
Dr. Angela Lanfranchi, M.D., F.A.C.S., explains that there is sufficient evidence to establish a cause and effect relationship between abortion and breast cancer. Scientists use six criteria to decide whether there is a causal relationship (and not just an association) between a particular factor and a disease. In the case of abortion-breast cancer research, all six criteria have been met. Speaking of the high bar of absolute proof of a link, that some individuals have erroneously demanded for this women's health issue, she said:
"The only proof would be if you took 100 closely matched women and impregnated them, aborted 50 of them and then followed them to see who got more breast cancer. Since that is an unethical thing to do to a human, we have been able to do that in rats. [Rat physiology is remarkably similar to human breast physiology] Russo and Russo 1980 showed that more aborted rats get breast cancer when exposed to a carcinogen than virginal rats or rats that have had litters of pups.
However, there are
SIX CRITERIA that epidemiologists use to determine if an association is in fact causal . Certainly, the abortion-breast cancer (ABC) studies show a causal relationship.
"First, the exposure or risk must precede the disease, which it does in all of the studies.
"Second, the preponderance of the studies must show an association. In the case of the ABC link, 28 out of 37 studies report an association between abortion and breast cancer.
"Third, the studies must include statistically significant studies. There are 17 statistically significant studies.
"Fourth, there must be a plausible biologic basis. In the case of the ABC link, if a pregnancy is terminated before 32 weeks, the woman is left with increased numbers of type 1 and 2 lobules which are most sensitive to carcinogens. It is only after 32 weeks [of pregnancy] that type 3 and 4 lobules resistant to carcinogens are formed.
It is the same reason why(Melbye et al. 1999 reported) women who have premature deliveries before 32 weeks more than double their risk of breast cancer and why women who have children have a lower risk of breast cancer. [2,3] Women who never have children are also at increased risk because of their lack of type 3 and 4 lobules.
"Fifth, there should be a dose effect, meaning the more you are exposed to a risk, the higher the risk. In the case of the ABC link, this is more difficult to show than the case of a drug that causes cancer. However, Melbye et al. 1997 showed that for every week you delay an abortion, the risk of breast cancer increases by 3 percent, so that his study showed a statistically significant increased risk of breast cancer among women with second trimester abortions. 
"Sixth, there must be a relative risk of over 3.0 or a 200% increased risk.
In subsets of women, the relative risk is greater than 3.0 :
Teenagers less than age 18 who have abortions between 9 and 24 weeks have an 800% increased risk or a relative risk of 9.0 according to the NCI commissioned study, Daling et al. 1994. This team found a relative risk of infinity among teenagers procuring abortions when they also had a family history of breast cancer. This was because all women in her study who had a family history and an abortion at age 18 or younger developed breast cancer by the age of 45. 
"As a breast surgeon, I became aware of younger and younger patients with breast cancer. When I had a large number of patients in their thirties with breast cancer, I analyzed my own data. Thirty percent of women in their thirties with breast cancer had no family history, but did have abortions. Whereas in my older patients, only 15% had had abortions. Every person in this country is aware that breast cancer is becoming a disease of young women. It is not a grandmother's disease any more."