Earlier large-scale studies showed no link between abortion and breast cancers that occur after
"The globality of evidence supports no link between induced abortion and breast cancer," Harvard researcher Karin Michels, ScD, PhD, tells WebMD.
The Michels study shows a longstanding "scientific consensus," says Michael Thun, MD, vice president for epidemiology and surveillance research at the American Cancer Society in Atlanta. Thun was not involved in the Michels study.
"There is no evidence that having had an abortion increases the risk of breast cancer," Thun tells WebMD. "This is a subject that has received a lot of visibility; something that has been looked at repeatedly. There is strong scientific consensus this is the case."
Abortion and Breast Cancer Risk
At one time, researchers did suspect a link between abortion and breast cancer. Researchers who asked women about their abortion history found that women with breast cancer were more likely than healthy women to report having had abortions.
But this kind of study -- called a case/control study -- is not considered particularly reliable. A person with a medical condition is more likely to report an unusual or embarrassing event than is a healthy person. That's particularly true when the event is an abortion.
"Abortion is such a personal and sensitive piece of information," Michel says. "If just you just ask random people, you get much more underreporting then you do when you ask women with breast cancer, who are much more likely to reveal all sorts of information if you ask them."
Three studies that looked at women's records about abortion before breast cancer developed found no link to breast cancer. Three studies that asked postmenopausal women about their abortion history and then observed them for long periods of time also found no link to breast cancer.
In 2003, the National Cancer Institute convened an expert panel to analyze these studies. It concluded that abortion did not affect breast cancer risk. Michels was a member of that panel.
"No study should be interpreted on its own," Michels warns. “This current study really supports the consensus that we came up with in 2003. So now we can close the loop and say the lack of abortion risk seen for postmenopausal breast cancer applies to premenopausal breast cancer as well."
Despite the scientific consensus, four states have laws on the books that require doctors to warn women seeking abortion that the procedure may cause breast cancer. Those states are Minnesota, Mississippi, Montana, and Texas. The law in Montana has been found unconstitutional and is not enforced.
This year, there were efforts to introduce similar laws in New Hampshire, North Dakota, and Wyoming, according to the Center for Reproductive Rights. All of these bills failed to pass into law.
Thirty-one states have laws that require "biased counseling and/or mandatory delays which may include providing information on breast cancer and abortion," a spokeswoman for Physicians for Reproductive Choice and Health tells WebMD.
The Michels study appears in the April 23 issue of Archives of Internal Medicine.