WASHINGTON Abortions and miscarriages do not raise the risk of breast cancer, despite claims by some groups and some studies that suggest they do, researchers said on Monday.
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 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 Brigham and Women's Hospital and Harvard Medical School in Boston.
But 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.
So to exercise control in a study for this, it is better to start with healthy women, get their answers on abortion and miscarriage first, and then watch to see who develops cancer, 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.
Breast cancer is far more common after menopause, but Michels' team noted that the studies that had seemed to show abortion caused breast cancer also mostly looked at younger women who had not reached menopause.
Researchers had reasons to suspect that abortion and miscarriage might possibly be linked to breast cancer. Women who give birth before the age of 35 have a lower risk of breast cancer.
Pregnancy causes hormonal changes, and one theory held that the interruption of the roller-coaster of hormones in the middle of a pregnancy might allow breast cells to turn cancerous.
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.