A new study at the Harvard Medical School, Boston, has confirmed that abortions and miscarries do not raise the risk of breast cancer.
A study of more than 100,000 U.S. nurses found that those who had an abortion or miscarriage were no more likely to have breast cancer than any other woman in the study.
The findings fit in with a 2003 report from an international expert panel put together by the U.S. National Cancer Institute.
"If you look at the high-quality evidence, it does not support an association between induced abortions and breast cancer," said Karin Michels of the Harvard school.
Her team set out to create the most reliable type of research that is possible -- a prospective study, starting with women before they ever had cancer, and following them for years.
Her team's study, published in the Archives of Internal Medicine, began with 105,000 women aged 29 to 46 years in 1993. All were cancer-free to start with and filled out a detailed, anonymous questionnaire that included questions about abortions and miscarriages.
Previous studies have started with women who already had breast cancer and asked them whether they had ever had an abortion.
Michels said women with cancer who have had an abortion are much more likely to report this. "They are still soul-searching and looking for reasons," Michels said in a telephone interview.
Abortion is one area that women are likely to keep quiet about, even in an anonymous survey, she said. "There will always be some underreporting because it is such a sensitive issue," Michels said.
They said 15 percent of the nurses reported they had ever had an abortion and 21 percent reported a miscarriage.
Over the 10 years of the study, 1,458 of all the women developed breast cancer, Michels' team found.
"Among this predominantly premenopausal population, neither induced nor spontaneous abortion was associated with the incidence of breast cancer; number of abortions, age at abortion, parity (having had a live baby) status, or timing of abortion with respect to a full-term pregnancy did not affect the results," they wrote.
The issue became political when, in 2002, the National Cancer Institute posted information potentially linking abortion and breast cancer on its Web site.
And last July, Democratic staff on the House of Representatives Government Reform Committee found that advisers working at some federally funded pregnancy resource centers gave out false information, telling callers there were strong and proven links between abortion and breast cancer.