ArticlesBreast cancer and abortion: collaborative reanalysis of data from 53 epidemiological studies, including 83 000 women with breast cancer from 16 countries
Introduction
Pregnancies that result in a birth are known to reduce a woman's long-term risk of developing breast cancer,1 but the effects of pregnancies that end as a spontaneous or, particularly, as an induced abortion are less clear, although many studies have relevant data.2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60 The findings from case-control studies—in which women were asked their abortion history after they were diagnosed with breast cancer—have been especially difficult to interpret. For, women who have had an induced abortion are known to under-report such events,61, 62, 63 but they might be more likely to disclose this information than they would otherwise have been if they had been diagnosed with breast cancer and knew that they were taking part in a research project investigating the causes of their disease.62 Hence, in case-control studies in which “cases” are women with recently diagnosed breast cancer and “controls” are women who do not have the disease, and in which information on past abortions is obtained retrospectively (ie, after the cases have been diagnosed with breast cancer), the extent of under-reporting of induced abortion might well differ between the cases and controls. For example, among women in a Swedish case-control study62 who had, in fact, had a previous induced abortion recorded on a national abortion register, 21% of those with breast cancer and 27% of those without the disease reported incorrectly that they had never had an induced abortion. Any such systematic differences between women with and without breast cancer in the under-reporting of past induced abortions could appreciably distort the results from studies with retrospectively recorded information on abortion,62 but could not similarly affect results from studies in which the information on abortion had been recorded prospectively, that is, before the diagnosis of breast cancer.
One of the goals of the Collaborative Group on Hormonal Factors in Breast Cancer, when it was set up in 1992, was to combine the worldwide epidemiological evidence on the relation between breast cancer and reproductive events, including pregnancies that end as abortions. Because, as noted above, differential retrospective reporting of induced abortion between cases and controls could have taken place, results of the studies with retrospectively obtained records of abortion are presented in this report separately from results of the studies with prospective records that predated the diagnosis of breast cancer.
Section snippets
Contributing studies and collection of data
Epidemiological studies were eligible for inclusion if they had (before any exclusions) 100 or more women with incident invasive breast cancer, were undertaken in countries with liberal abortion laws,61 and had systematically sought information about every woman on reproductive history, including details about previous spontaneous and induced abortions. Potentially eligible studies were identified from review articles, computer-aided literature searches done up to October, 2003, with MEDLINE,
Results
The table contains details of the 53 studies from 16 countries that were available for analysis, which contribute a total of 83 000 women with breast cancer. Studies are grouped according to whether information on abortion was recorded prospectively (44 000 women with breast cancer) or retrospectively (39 000 women with breast cancer). Within each type of study design, individual studies are listed chronologically, according to the median year of diagnosis of breast cancer in the study
Discussion
An advantage of seeking to review all available studies of breast cancer and abortion is that this helps avoid unduly selective emphasis on particular studies, or just on published results. Only about two-thirds of the eligible studies that had obtained relevant information had published their findings on abortion and breast cancer.2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45
References (85)
- et al.
Consistent lack of association between breast cancer and oral contraceptives using either hospital or neighbourhood controls
Prev Med
(1993) - et al.
Alcohol and breast cancer in the Swiss Canton of Vaud
Eur J Cancer
(1996) Abortion and breast cancer: a hard decision made harder
Lancet Oncol
(2001)Breast cancer and breastfeeding: collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50 302 women with breast cancer and 96 973 women without the disease
Lancet
(2002)- et al.
Oral contraceptive use and early abortion as risk factors for breast cancer in young women
Br J Cancer
(1981) - et al.
Reproductive factors in the aetiology of breast cancer
Br J Cancer
(1983) - et al.
Oral contraceptive use and breast cancer or cervical cancer: preliminary results of a French case-control study
Hormones and sexual factors in human cancer aetiology
(1984) - et al.
Relationship between risk factors for breast cancer and hormonal status
Int J Epidemiol
(1984) - et al.
Role of fat, animal protein and dietary fibre in breast cancer aetiology: a case-control study
J Natl Cancer Inst
(1986) - et al.
Early oral contraceptive use and breast cancer: results of another case-control study
Br J Cancer
(1987)
Risk of breast cancer in relation to reproductive factors in Denmark
Br J Cancer
Breast cancer and the pill: a further report from the Royal College of General Practitioners' oral contraception study
Br J Cancer
A case-control study of breast cancer in relation to oral contraceptive use in Slovenia
Neoplasma
Oral contraceptive agents and breast cancer: a population-based case-control study
Med J Aust
Risk factors for breast cancer in Chinese women in Shanghai
Cancer Res
Oral contraceptive use and breast cancer risk in young women
Lancet
Oral contraceptives and breast cancer: latest findings in a large cohort study
Br J Cancer
Absence of and association between reproductive variables and risk of breast cancer in young women in Sweden and Norway
Br J Cancer
Oral contraceptives and breast cancer: a French case-control study
Int J Epidemiol
Spontaneous and induced abortion and risk of breast cancer
Int J Cancer
Varma AO. Breast cancer risk and oral contraceptive use: results from a large case-control study
Epidemiology
Age-specific differences in the relationship between oral contraceptive use and breast cancer
Obstet Gynecol
A case-control study of the risk of breast cancer in relation to oral contraceptive use
Am J Epidemiol
A case-control study of breast cancer in Tianjin, China
Cancer Epidemiol Biomarkers Prev
Non-contraceptive hormone use and risk of breast cancer
Cancer Causes Control
Breast cancer and prolonged lactation
Int J Epidemiol
Risk of breast cancer among young women: relationship to induced abortion
J Natl Cancer Inst
Abortion and the risk of breast cancer: a case-control study in Greece
Int J Cancer
Breast cancer risk and oral contraceptive use in Slovenian women aged 25 to 54
Int J Cancer
Risk of breast cancer risk among white women following induced abortion
Am J Epidemiol
Pregnancy termination in relation to risk of breast cancer
JAMA
Induced abortion and risk for breast cancer: reporting (recall) bias in a Dutch case-control study
J Natl Cancer Inst
Oral contraceptive use and risk of breast cancer in middle-aged women
Am J Epidemiol
Abortion and breast cancer risk
Int J Cancer
Induced abortion and the risk of breast cancer
N Engl J Med
Reversal of relation between body mass and endogenous estrogen concentrations with menopausal status
J Natl Cancer Inst
Breastfeeding experience and breast cancer risk among postmenopausal women
Cancer Epidemiol Biomarkers Prev
Breast cancer in Australian women under the age 40
Cancer Causes Control
Reproductive factors and breast cancer in New Zealand
Int J Cancer
Design and analysis issues in a population-based, case-control-family study of the genetic epidemiology of breast cancer and the Co-operative Family Registry for Breast Cancer Studies (CFRBCS)
J Natl Cancer Inst Monogr
Adolescent reproductive events and subsequent breast cancer risk
Am J Public Health
Induced abortion and breast cancer risk
Epidemiology
Cited by (0)
Collaborators listed at http://image.thelancet.com/extras/04art2192webappendix.pdf