Elsevier

The Lancet

Volume 363, Issue 9414, 27 March 2004, Pages 1007-1016
The Lancet

Articles
Breast cancer and abortion: collaborative reanalysis of data from 53 epidemiological studies, including 83 000 women with breast cancer from 16 countries

https://doi.org/10.1016/S0140-6736(04)15835-2Get rights and content

Summary

Background

The Collaborative Group on Hormonal Factors in Breast Cancer has brought together the worldwide epidemiological evidence on the possible relation between breast cancer and previous spontaneous and induced abortions.

Methods

Data on individual women from 53 studies undertaken in 16 countries with liberal abortion laws were checked and analysed centrally. Relative risks of breast cancer—comparing the effects of having had a pregnancy that ended as an abortion with those of never having had that pregnancy—were calculated, stratified by study, age at diagnosis, parity, and age at first birth. Because the extent of under-reporting of past induced abortions might be influenced by whether or not women had been diagnosed with breast cancer, results of the studies—including a total of 44000 women with breast cancer—that used prospective information on abortion (ie, information that had been recorded before the diagnosis of breast cancer) were considered separately from results of the studies—including 39000 women with the disease—that used retrospective information (recorded after the diagnosis of breast cancer).

Findings

The overall relative risk of breast cancer, comparing women with a prospective record of having had one or more pregnancies that ended as a spontaneous abortion versus women with no such record, was 0·98 (95% CI 0·92–1·04, p=0·5). The corresponding relative risk for induced abortion was 0·93 (0·89–0·96, p=0·0002). Among women with a prospective record of having had a spontaneous or an induced abortion, the risk of breast cancer did not differ significantly according to the number or timing of either type of abortion. Published results on induced abortion from the few studies with prospectively recorded information that were not available for inclusion here are consistent with these findings. Overall results for induced abortion differed substantially between studies with prospective and those with retrospective information on abortion (test for heterogeneity between relative risks: χ21 = 33·1, p<0·0001).

Interpretation

Pregnancies that end as a spontaneous or induced abortion do not increase a woman's risk of developing breast cancer. Collectively, the studies of breast cancer with retrospective recording of induced abortion yielded misleading results, possibly because women who had developed breast cancer were, on average, more likely than other women to disclose previous induced abortions.

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

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