Opponents of the abortion-breast cancer link do not challenge the biological reason for the link. They challenge the evidence showing a statistical correlation -- the epidemiological research.Read more.
Epidemiology is the study of disease trends in large populations. By itself, epidemiology cannot establish whether or not a cause-effect relationship exists, but it can be used as one more piece in the puzzle that supports a cause-effect relationship.
Seventy epidemiological studies have been conducted in Asia, Europe, Australia and the U.S. since 1957. Eighty percent of these studies report risk increases for women who had abortions.
The National Cancer Institute specifically commissioned a study by Janet Daling and her colleagues. She reported that, "Among women who had been pregnant at least once, the risk of breast cancer in those who had experienced an induced abortion was 50% higher than among other women." "Highest risks (more than double) were observed when the abortion was done at ages younger than 18 years ... or at least thirty years of age or older."
Daling et al. identified several high-risk groups, i.e. teenagers who have abortions before age 18, women with a family history of the disease and women 30 years of age or older.
Professor Joel Brind of Baruch College and his co-authors at Penn State conducted a scientific review and meta-analysis of 23 studies. They found 18 that reported risk increases. They reported a 30% risk increase for women who have abortions after the birth of a first child and a 50% risk increase for women who have abortions before the birth of a first child.