Largest US study yet finds no link between abortion and risk of breast cancer

A large scale study has found no evidence of a link between incomplete pregnancy, such as that from induced abortion, and risk of breast cancer. The new research, published in the June issue of Contraception is set to draw a line under a long contentious issue, claim the researchers.

The study, undertaken at the Department of Cancer Etiology, City of Hope Comprehensive Cancer Center, Duarte, California, adds to mounting evidence from similar large prospective studies, which have yet to quell all public concerns, explained lead investigator Katherine DeLellis Henderson .

She added; "much of the data prompting this concern came from case-control studies, many of which may have been affected by bias or design flaws," reported Medwire News.

There are two main mechanisms hypothesized to underlie a potential association between incomplete pregnancy and breast cancer risk.

First, women who have an incomplete pregnancy may not experience the long-term protection against breast cancer that a full-term pregnancy would provide.

Second, the breasts of women who have an incomplete pregnancy are exposed to high hormone levels typical of early normal pregnancy, but then do not experience the terminal cell differentiation that occurs late in a normal pregnancy, possibly leaving breast tissue more vulnerable to carcinogens.

Henderson and colleagues tested these theories in 109,893 women enrolled in the 1995 California Teachers Study, a large prospective cohort with detailed pregnancy history data.

Overall 21,168 women in this cohort reported ever having had miscarriage while 15,126 women reported having an induced abortion.

After an average follow-up of 9 years there were 3325 cases of incident invasive breast cancer diagnosed.

After adjusting for established risk factors including ethnicity, first-degree family history of breast cancer, and age at menarche, having an induced abortion in a first pregnancy did not increase a woman's risk for breast cancer among women who went on to have full-term pregnancies or those who never went on to give birth. Similarly, miscarriage in a first pregnancy did not increase breast cancer risk in either group of women. They used a statistical tool known as Cox multivariable regression and found no statistically significant link between any measure of incomplete pregnancy and breast cancer risk.

The researchers said their findings supported those from a recent large scale study of nurses, the Nurses Health Study II, concluding that:

"Our results provide further, strong evidence that neither induced abortion nor miscarriage is associated with breast cancer risk and may help to resolve any remaining uncertainty as to whether such a relationship exists."

For more information: Contraception 2008; 77: 391-396