A new US study suggests that breast cancer risk is not increased by induced abortion or miscarriage, contrary to the findings of some other studies and the claims made by some groups.
The study is published in the Archives of Internal Medicine.
Researchers from Brigham and Women’s Hospital, Harvard Medical School and the Harvard School of Public Health, Boston in Massachusetts, used data from the Nurses’ Health Study II (NHS II) on 105,716 women who were aged between 29 and 46 years and cancer-free at the start of the ten-year follow-up which started in 1993.
The cohort is described by the researchers as “predominantly premenopausal”, since most of the women had not started the menopause during the follow-up.
Records for the large scale prospective study were updated by questionnaire response every two years, and contained information on spontaneous abortions (miscarriages) and induced abortions, including any that had occurred before the period of study.
A large scale prospective study is considered high standard for this kind of research because you start with a large group of healthy participants and you monitor them for a good part of their lives to see who develops the symptoms under investigation.
This study covered 973,437 person-years of follow-up between 1993 and 2003.
(The number of person years of a study is an indication of its size and it is not unusual for researchers to talk about events per person year, which is the total number of events being counted divided by the total number of years of data from each participant.)
The results showed that:
— 1,458 new cases of invasive breast cancer occurred during the follow-up.
— 16,118 participants (15 per cent) reported a history of induced abortion.
— 21,753 (21 per cent) reported a history of spontaneous abortion (miscarriage).
— Neither abortion nor miscarriage was linked significantly with breast cancer.
— The results were unaffected by number of abortions or miscarriages, age of woman when the events occurred, and other factors.
In summary, the researchers found no significant link between abortion, miscarriage and breast cancer across the group. However, they did report two subgroup effects.
One subgroup showed a link between induced abortion and progesterone receptor-negative breast cancer, the kind of cancer that does not respond to the hormone progesterone.
The other subgroup showed a link between miscarriage before the age of 20 and lower breast cancer risk.
But in both cases the researchers suspect the results are spurious, due to chance, because the numbers in the two groups are too low to give a confident statistical finding.
Their overall conclusion therefore was that:
“Among this predominantly premenopausal population, neither induced nor spontaneous abortion was associated with the incidence of breast cancer.”
About the Nurses’ Health Study II (NHS II)
The NHS II is a prospective study cohort of over 116,000 women enrolled in 1989.
The purpose of the NHS II study is to look at the link between potentially modifiable risk factors and any major health problems that arise among women in early adult life. Breast cancer and melanoma are two particular areas of interest to the researchers.
The participants fill in questionnaires about their lifestyle, dietary habits, life events, exercise, use of contraceptives, and health issues every two years.
Adverse health events such as cancer diagnosis are followed up with more detailed investigation of medical records, and blood samples are analysed from about 30,000 of the cohort.
90 per cent response rate has been maintained for the questionnaire follow-ups.
“Induced and Spontaneous Abortion and Incidence of Breast Cancer Among Young Women: A Prospective Cohort Study.”
Karin B. Michels, Fei Xue, Graham A. Colditz, and Walter C. Willett.
Arch Intern Med. 2007;167:814-820.
Vol. 167 No. 8, April 23, 2007
Written by: Catharine Paddock
Writer: Medical News Today