Abortion and breast cancer risk in seven countries

From Cancer Causes and Control
Issue: Vol 6 Number 1 January 1995
Title: Abortion and breast cancer risk in seven countries Authors: K B Michels (to whom correspondence should be addressed), C-c Hsieh, D Trichopoulos, and W C Willett

Ms Michels and Drs Hsieh, Trichopoulos, and Willett are with the Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115, USA. Dr Willett is also affiliated with the Department of Nutrition, Harvard School of Public Health, and Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA.

Epidemiologic studies have been inconsistent in suggesting an association between abortion and breast cancer risk. Whether the protection provided by a full-term pregnancy also results from a short-term pregnancy or whether a prematurely terminated pregnancy could increase the risk of breast cancer is unclear. Data from a large, international collaborative study were used to evaluate the association between abortions, whether spontaneous or induced, and breast cancer risk. The data from seven countries included 3,958 breast cancer cases and 11,538 hospital controls with information on abortion history obtained through interviews.

Compared with nulliparous women with no abortion (baseline), the odds ratios (OR) and 95 percent confidence intervals (CI) were: for nulliparous women with a history of prior abortion, 0.86 (CI = 0.68-1.08); for parous women with no history of abortion, 0.63 (CI = 0.57-0.69); for parous women with abortion before first birth, 0.82 (CI = 0.69-0.97); and, for parous women with abortion only after first birth, 0.70 (CI = 0.63-0.79). When restricting analysis to parous women, those with a history of abortion exhibited an elevated OR suggesting a 29 percent risk increase if the incomplete pregnancy occurred before first birth (CI = 1.16-1.36) and an 11 percent risk increase for abortion only after first birth (CI = 1.02-1.20) compared with women without such history.

The associations observed were stronger among the youngest women. These results do not support a large overall association between abortion and breast cancer risk.

Cancer Causes and Control 1995, 6, 75-82.

Key words: Abortion, breast cancer, international comparison, miscarriage, pregnancy.

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