Response bias in a case-control study: analysis utilizing comparative data concerning legal abortions from two independent Swedish studies

Time for Better Access to High-Quality Abortion Data in the United States

Despite considerable lay attention on the regulation and legislation of abortion in the United States, important gaps remain in our understanding of its incidence and health and social consequences since its legalization in 1973. Many of these gaps in knowledge can be attributed to a lack of access to high-quality, individual-level abortion data over the past 46 years. Herein, we review the strengths and limitations of different, currently available methods for enumerating abortions in the United States and discuss how lack of access to high-quality data limits our surveillance and research activities of not only abortion but other important reproductive and perinatal health outcomes. We conclude by discussing some potential opportunities for improved access to high-quality abortion data in the United States.

Pregnancy duration and endometrial cancer risk: nationwide cohort study

OBJECTIVE

To explore the association between pregnancy duration and risk of endometrial cancer.

DESIGN

Nationwide register based cohort study.

SETTING

Denmark.

PARTICIPANTS

All Danish women born from 1935 to 2002.

MAIN OUTCOME MEASURES

Relative risk (incidence rate ratio) of endometrial cancer by pregnancy number, type, and duration, estimated using log-linear Poisson regression.

RESULTS

Among 2 311 332 Danish women with 3 947 650 pregnancies, 6743 women developed endometrial cancer during 57 347 622 person years of follow-up. After adjustment for age, period, and socioeconomic factors, a first pregnancy was associated with a noticeably reduced risk of endometrial cancer, whether it ended in induced abortion (adjusted relative risk 0.53 (95% confidence interval 0.45 to 0.64) or childbirth (0.66, 0.61 to 0.72). Each subsequent pregnancy was associated with an additional reduction in risk, whether it ended in induced abortion (0.81, 0.77 to 0.86) or childbirth (0.86, 0.84 to 0.89). Duration of pregnancy, age at pregnancy, spontaneous abortions, obesity, maternal birth cohort, fecundity, and socioeconomic factors did not modify the results.

CONCLUSIONS

The risk of endometrial cancer is reduced regardless of whether a pregnancy ends shortly after conception or at 40 weeks of gestation. This reduction in risk could be explained by a biological process occurring within the first weeks of pregnancy, as pregnancies ending in induced abortions were associated with similar reductions in risk as pregnancies ending in childbirth.

Associations of reproductive time events and intervals with breast cancer risk: a report from the Shanghai Breast Cancer Study

While there is clear evidence for an association between later age at first live birth and increased breast cancer risk, associations with the timing of other reproductive events are less clear. As breast tissues undergo major structural and cellular changes during pregnancy, we examined associations between reproductive time events and intervals with breast cancer risk among parous women from the population-based Shanghai Breast Cancer Study (SBCS). Unconditional logistic regression was used to evaluate associations with breast cancer risk for 3,269 cases and 3,341 controls. In addition to later age at first live birth, later ages at first pregnancy and last pregnancy were significantly associated with increased breast cancer risk (p-trend = 0.002, 0.015, 0.008, respectively); longer intervals from menarche to first or last live birth were also associated with increased risk (p-trend < 0.001, =0.018, respectively). Analyses stratified by menopausal status and estrogen receptor (ER)/progesterone receptor (PR) status revealed that associations for later age at first pregnancy or live birth and longer intervals from menarche to first or last live birth occurred among premenopausal women and ER+/PR+ breast cancers, whereas the association for later age at last pregnancy occurred among postmenopausal women and women with ER+/PR- or ER-/PR+ breast cancers. Because of the high correlation with other reproductive variables, models did not include adjustment for age at first live birth; when included, the significance of all associations was attenuated. These findings suggest that while reproductive time events and intervals play an important role in breast cancer etiology, contributions may differ by menopausal status and hormone receptor status of breast cancers.

Fertility treatments, congenital malformations, fetal loss, and childhood acute leukemia: the ESCALE study (SFCE)

BACKGROUND

This study investigated the relationships between childhood acute leukemia (AL) and selective maternal and birth characteristics, including congenital malformations and the use of fertility treatment, for which the literature remains scarce.

PROCEDURE

The national registry-based case-control study ESCALE was carried out in France in 2003-2004. Population controls were frequency matched with cases on age and gender. Data were obtained from structured telephone questionnaires. Odds ratios (OR) and their 95% confidence intervals were estimated using unconditional regression models adjusted for potential confounders.

RESULTS

In all, 764 cases of AL (648 lymphoblastic AL (acute lymphoblastic leukemia, ALL) and 101 myeloblastic AL) and 1,681 controls were included. The AL cases' mothers reported congenital malformations more frequently than the controls' mothers (OR = 1.5 [1.0-2.4]). ALL was significantly associated with the use of fertility treatment for the index pregnancy (OR = 1.9 [1.3-2.8]). In particular, ALL was associated with ovulation induction only (OR = 2.6 [1.6-4.3]), but not with in vitro fertilization (IVF, OR = 1.0 [0.4-2.3]) or artificial insemination (OR = 1.3 [0.5-3.9]). A positive association was also observed for the difficulty of becoming pregnant without fertility treatment (OR = 1.5 [1.0-2.1]). AL was positively associated with a history of voluntary abortion (OR = 1.4 [1.1-1.8]) but not with a history of spontaneous (OR = 0.8 [0.7-1.0]) or therapeutic (OR = 0.7 [0.5-1.1]) abortion.

CONCLUSION

The results suggest that subfertility in itself and ovulation induction may be associated with ALL, and support a positive association with congenital malformations. The links with the various types of fertility drugs and the underlying causes of infertility need to be investigated further.

False alarms and pseudo-epidemics: the limitations of observational epidemiology

Most reported associations in observational clinical research are false, and the minority of associations that are true are often exaggerated. This credibility problem has many causes, including the failure of authors, reviewers, and editors to recognize the inherent limitations of these studies. This issue is especially problematic for weak associations, variably defined as relative risks (RRs) or odds ratios (ORs) less than 4. Such associations, commonly reported in the medical literature, are more likely to be attributable to bias than to causal association. All observational research has bias (which can include selection, information, and confounding bias). Hence, detection of small associations falls below the discriminatory ability of observational studies. In general, unless RRs in cohort studies exceed 2 to 3 or ORs in case-control studies exceed 3 or 4, associations in observational research findings should not be considered credible. However, these guidelines are not foolproof: strong (yet spurious) associations can result when large amounts of bias are present. Only in a properly performed randomized controlled trial, free of bias, should small associations merit attention. Better training and more circumspection on the part of investigators, tougher editorial standards on the part of journals, and hefty skepticism on the part of referees and readers are necessary to avoid the dangers of false alarms, pseudo-epidemics, and their unfortunate consequences.

Incomplete pregnancy and risk of ovarian cancer: results from two Australian case-control studies and systematic review

Although full-term pregnancies reduce the risk of ovarian cancer, it has not been conclusively established whether incomplete pregnancies also influence risk. We investigated the relationship between a history of incomplete pregnancy and incident epithelial ovarian cancer among over 4,500 women who participated in two large Australian population-based case-control studies in 1990-1993 and 2002-2005. They provided responses to detailed questions about their reproductive histories and other personal factors. Summary odds ratios (OR) and confidence intervals (CI) derived for each study using the same covariates were aggregated. We found no significant associations between the number of incomplete pregnancies and ovarian cancer, for parous (OR = 0.98, 95% CI: 0.89, 1.08) or nulliparous (OR = 1.06, 95% CI: 0.75, 1.48) women, nor for the number of spontaneous or induced abortions and ovarian cancer for parous women (OR = 0.95, 95% CI 0.82, 1.09; OR = 1.08, 95% CI: 0.86, 1.36) or nulliparous women (OR = 1.2, 95% CI: 0.6, 2.4; OR = 0.8, 95% CI: 0.47, 1.38), respectively. A systematic review of 37 previous studies of the topic confirmed our findings that a history of incomplete pregnancy does not influence a woman's risk of epithelial ovarian cancer.

Social desirability bias in family planning studies: a neglected problem

Studies on family planning methods traditionally have relied on self-reports of unknown validity and reproducibility. Social desirability bias, a type of information bias, occurs when study participants respond inaccurately - but in ways that will be viewed favorably by others. Several lines of evidence reveal that this bias can be powerful in sexual matters, including reports of coitus, use of contraceptives and induced abortion. For example, studies using vaginal prostate-specific antigen testing reveal underreporting of unprotected coitus and overreporting of barrier contraceptive use. Medication Event Monitoring System studies, which electronically record the time of pill dispensing from a bottle or pack, indicate widespread exaggeration of adherence to pill-taking regimens, including oral contraceptives. Comparisons of provider data and self-reports of induced abortions reveal extensive underreporting of induced abortion. Reliance on self-reported data underestimates contraceptive efficacy. Although techniques to minimize this bias exist, they are infrequently used in family planning studies. Greater skepticism about self-reports and more objective means of documenting coitus and contraceptive use are needed if contraceptive efficacy is to be accurately measured.

False-positive results in cancer epidemiology: a plea for epistemological modesty

False-positive results are inherent in the scientific process of testing hypotheses concerning the determinants of cancer and other human illnesses. Although much of what is known about the etiology of human cancers has arisen from well-conducted epidemiological studies, epidemiology has been increasingly criticized for producing findings that are often sensationalized in the media and fail to be upheld in subsequent studies. Herein we describe examples from cancer epidemiology of likely false-positive findings and discuss conditions under which such results may occur. We suggest general guidelines or principles, including the endorsement of editorial policies requiring the prominent listing of study caveats, which may help reduce the reporting of misleading results. Increased epistemological humility regarding findings in epidemiology would go a long way to diminishing the detrimental effects of false-positive results on the allocation of limited research resources, on the advancement of knowledge of the causes and prevention of cancer, and on the scientific reputation of epidemiology and would help to prevent oversimplified interpretations of results by the media and the public.

Case-control study of risk factors for breast cancer in Nigerian women

This study evaluated the potential risk factors for breast cancer in Nigerian women using a case-control design of 250 women with breast cancer and their age-matched female controls. Both cases and controls were recruited from 4 University Teaching Hospitals in Midwestern and Southeastern Nigeria. Data on the clinical and epidemiological characteristics were collected using interviewer-administered structured questionnaires. The mean age of the cases and controls were 46.1 and 47.1 years, respectively. Fifty-seven percent of the cases were premenopausal while 43% were postmenopausal. The association of risk factors with breast cancer was assessed using conditional logistic regression. Positive family history of breast cancer in first- and second-degree relatives (Odds ratio [OR] = 8.07, 95% confidence interval [CI], 1.003, 64.95, p = 0.04), education of high school level and above (OR = 1.35, 95% CI 1.04, 1.74, p = 0.0205), age at first fullterm pregnancy (FFTP) greater than 20 years (OR = 1.32 95% CI 1.01, 1.71, p = 0.0413) and waist/hip ratio (WHR) (OR = 1.98, 95% CI 1.27, 3.10, p = 0.0026) were associated with increased risk of breast cancer in the final multiple conditional logistic regression model. The findings from this study have shown that sociodemographic characteristics, reproductive variables and anthropometric measures are significant predictors of breast cancer risk in Nigerian women.

Breast cancer risk in relation to abortion: Results from the EPIC study

The role of spontaneous and induced abortion on breast cancer risk is examined among 267,361 women recruited into the European Prospective Investigation into Cancer and nutrition between 1992 and 2000. The data were collected from 20 centers, across 9 countries, and included information on a total of 4,805 women with breast cancer, of whom 1,657 reported having ever had any type of abortion. Overall, the relative risk of breast cancer in women who reported ever having had a spontaneous abortion was not significantly elevated when compared with women who reported never having had such an abortion (RR = 1.07, 95% CI = 0.99-1.14). However, there was some evidence of a slight increase in the risk of breast cancer among women who reported having had 2 or more spontaneous abortions (1.20, 1.07-1.35). The relative risk of breast cancer among women who reported ever having had an induced abortion when compared to women who reported never having had an induced abortion was 0.95 (0.87-1.03). Overall, the findings provide further unbiased evidence of the lack of an adverse effect of induced abortion on breast cancer risk.

Risk of breast cancer after miscarriage or induced abortion: a Scottish record linkage case-control study

STUDY OBJECTIVE

To assess the risk of breast cancer in patients with a previous history of miscarriage or induced abortion.

DESIGN

Case-control study relating "exposure" to outcome by linkage of national hospital discharge and maternity records, the national cancer registry, and death records.

SETTING

Scotland.

PARTICIPANTS

Miscarriage analysis-2828 women with breast cancer and 9781 matched controls; induced abortion analysis-2833 women with breast cancer and 9888 matched controls.

MAIN RESULTS

After stratification for age at diagnosis, parity, and age at first birth, the odds ratio (95% confidence intervals) of breast cancer was 1.02 (0.88 to 1.18) in women with a previous miscarriage, and 0.80 (0.72 to 0.89) in women with a previous induced abortion. Further adjustments for age at bilateral oophorectomy, socioeconomic status (based on small area of residence), and health board area of residence had only minor effects on these odds ratios.

CONCLUSION

These data do not support the hypothesis that miscarriage or induced abortion represent substantive risk factors for the future development of breast cancer.

Memory decay and performance-related information bias in the reporting of scores by event riders

We used data from a case-control study investigating risk factors for horse falls in the cross-country phase of eventing in Great Britain (GB) to examine evidence for memory decay and information bias. Responses to two questions obtained by telephone for 173 cases and 521 controls were examined for evidence of differential reporting according to the respondent's case-control status and performance in the dressage and cross-country phases of competitions. Information bias was found in the accuracy of reporting dressage penalty scores when analysed as a function of performance level (good/poor). Poor dressage performers were less likely to report accurate dressage scores than good performers. The accuracy of reporting dressage scores decreased as the time between the event and questionnaire completion increased, with no case-control interaction. Competitors who incurred cross-country jumping penalties at the event preceding the selected event reported their cross-country scores with less accuracy when compared with competitors who incurred no penalties. No information bias was found when the reporting of dressage and cross-country scores were analysed as a function of respondent category (case/control).

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

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: chi2(1) =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.

The association between induced and spontaneous abortion and risk of breast cancer in Slovenian women aged 25-54

The risk of breast cancer may be increased by induced or spontaneous abortion. The evidence for this association was evaluated in a population based case-control study in Slovenia, where 624 women aged 25-54 years with breast cancer diagnosed during 1988-1990 were matched for age and site of residence with controls randomly selected from the Slovenian Population Registry. Odds ratios (OR) and 95% confidence intervals (CI) were obtained by conditional logistic regression analyses. Spontaneous abortion was not associated with a significantly increased risk of breast cancer (nulliparous women: OR=1.41, 95% CI 0.22-9.01; uniparous women: OR=0.98, 95% CI 0.50-1.91; women with parity 2 or more: OR=1.40, 95% CI 0.91-2.15). Induced abortion was not associated with a statistically significant elevated risk. The risk of breast cancer was higher in nulliparous women (OR=2.49, 95% Cl 0.68-9.09), and was less among women who had more deliveries. In uniparous women, the risk of breast cancer appeared higher when the induced abortion took place before a first full-term pregnancy (OR=1.94, 95% CI 0.70-5.39) rather than after a first full-term pregnancy (OR=1.22, 95% CI 0.71-2.10) but neither of these odds ratios reached significance. We found no significant association between spontaneous abortion or induced abortion and breast cancer risk. This study found an elevated, but not statistically significant, risk associated with induced abortion among nulliparous women and among parous women when the induced abortion was before the first full-term pregnancy.

Null association between pregnancy termination and breast cancer in a registry-based study of parous women

Studies suggesting a positive association between pregnancy termination and breast cancer risk have often been of retrospective case-control design, so subject to selection and recall biases. We undertook a registry-based analysis with minimal selection bias and prospective record-based ascertainment of terminations. The source population comprised Massachusetts women with a record of giving birth between 1987 and 1999 in the Massachusetts Vital Statistics Registry. Primary breast cancer cases were 25-55 years old at diagnosis between 1988 and 2000 and had a record of the diagnosis in the Massachusetts Cancer Registry. We matched 3 controls to each case on maternal age, year of giving birth and birth facility. Information on terminations (induced and spontaneous) before the birth of record, the matched factors and potential confounders were collected from the birth certificate. After adjustment for the matched factors, age, parity and maternal and paternal education, the odds ratio associating any termination history with breast cancer risk equaled 0.91 (95% CI = 0.79-1.05). The marginally protective adjusted odds ratio largely derived from a protective effect among women with parity equaled to 1 (OR for any termination = 0.68; 95% CI = 0.45-1.03), suggesting a protective effect of terminated pregnancy among women with one live birth.

Long-term physical and psychological health consequences of induced abortion: review of the evidence

Induced abortion is a prevalent response to an unintended pregnancy. The long-term health consequences are poorly investigated and conclusions must be drawn from observational studies. Using strict inclusion criteria (study population >100 subjects, follow up >60 days) we reviewed an array of conditions in women's health. Induced abortion was not associated with changes in the prevalence of subsequent subfertility, spontaneous abortion, or ectopic pregnancy. Previous abortion was a risk factor for placenta previa. Moreover, induced abortion increased the risks for both a subsequent preterm delivery and mood disorders substantial enough to provoke attempts of self-harm. Preterm delivery and depression are important conditions in women's health and avoidance of induced abortion has potential as a strategy to reduce their prevalence. Only review articles including the single published meta-analysis exploring linkages between abortion and breast cancer were relied upon to draw conclusions. Reviewers were mixed on whether subsequent breast neoplasia can be linked to induced abortion, although the sole meta-analysis found a summary odds ratio of 1.2. Whatever the effect of induced abortion on breast cancer risk, a young woman with an unintended pregnancy clearly sacrifices the protective effect of a term delivery should she decide to abort and delay childbearing. That increase in risk can be quantified using the Gail Model. Thus, we conclude that informed consent before induced abortion should include information about the subsequent risk of preterm delivery and depression. Although it remains uncertain whether elective abortion increases subsequent breast cancer, it is clear that a decision to abort and delay pregnancy culminates in a loss of protection with the net effect being an increased risk.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians.

LEARNING OBJECTIVES

After completion of this article, the reader will be able to define the terms and, to outline the epidemiologic problems in studying the long-term consequences of abortion, and to list the associated long-term consequences of abortion.

Breast cancer in relation to induced abortions in a cohort of Chinese women

The possible influence of induced abortion on breast cancer risk was assessed in a cohort of 267 040 women enrolled in a randomised trial of breast self-examination in Shanghai, China. Based on answers to a baseline questionnaire, subsequent breast cancer risk was not significantly associated with ever having an induced abortion. After adjustment for potential confounders, the relative risk estimate was 1.06 (95% C.I.: 0.91, 1.25), and there was no trend in risk with number of abortions. Analysis of data from more detailed interviews of 652 cases and 694 controls from the cohort yielded similar results. There was also no overall increase in risk in women with induced abortion after first birth. Few women had undergone an abortion after 13 weeks gestation or before their first child. Although increases in risk were observed in such women, they were not statistically significant and could have been due to recall bias. Abortions as they have been performed in China are not an important cause of breast cancer.

A comparison of responses to a health and lifestyle questionnaire completed before and then after blood pressure screening

Changes over a 2-week period in self-reported health and lifestyle information were examined and related to medical feedback provided prior to the second self-report. The study group consisted of 128 females and 73 males ranging in age from 35 to 85 with a mean age of 55.5 years. On two occasions subjects completed self-assessment questionnaires including 52 items covering symptomatology and various lifestyle behaviours. Before the second self-assessment, subjects were classified as cases if their blood pressure was borderline or hypertensive according to World Health Organization guidelines or as controls otherwise. Subjects were informed of this classification. Across questions, the percentage disagreement between reports ranged from 0.0% to 44.1% with a median of 17.5% and with case-control differences significant at the 10% level for four questions. The mean difference between responses was significantly different from 0 at the 10% level for 15 questions with case-control differences significant at the 10% level for 6 questions. For 2 questions, the significance level of the association of response with case-control status changed from a value greater than 20% based on responses before feedback to a value less than 5% based on responses after feedback. Some evidence of recall bias was found.

Case-control studies: research in reverse

Epidemiologists benefit greatly from having case-control study designs in their research armamentarium. Case-control studies can yield important scientific findings with relatively little time, money, and effort compared with other study designs. This seemingly quick road to research results entices many newly trained epidemiologists. Indeed, investigators implement case-control studies more frequently than any other analytical epidemiological study. Unfortunately, case-control designs also tend to be more susceptible to biases than other comparative studies. Although easier to do, they are also easier to do wrong. Five main notions guide investigators who do, or readers who assess, case-control studies. First, investigators must explicitly define the criteria for diagnosis of a case and any eligibility criteria used for selection. Second, controls should come from the same population as the cases, and their selection should be independent of the exposures of interest. Third, investigators should blind the data gatherers to the case or control status of participants or, if impossible, at least blind them to the main hypothesis of the study. Fourth, data gatherers need to be thoroughly trained to elicit exposure in a similar manner from cases and controls; they should use memory aids to facilitate and balance recall between cases and controls. Finally, investigators should address confounding in case-control studies, either in the design stage or with analytical techniques. Devotion of meticulous attention to these points enhances the validity of the results and bolsters the reader's confidence in the findings.

Bias and causal associations in observational research

Readers of medical literature need to consider two types of validity, internal and external. Internal validity means that the study measured what it set out to; external validity is the ability to generalise from the study to the reader's patients. With respect to internal validity, selection bias, information bias, and confounding are present to some degree in all observational research. Selection bias stems from an absence of comparability between groups being studied. Information bias results from incorrect determination of exposure, outcome, or both. The effect of information bias depends on its type. If information is gathered differently for one group than for another, bias results. By contrast, non-differential misclassification tends to obscure real differences. Confounding is a mixing or blurring of effects: a researcher attempts to relate an exposure to an outcome but actually measures the effect of a third factor (the confounding variable). Confounding can be controlled in several ways: restriction, matching, stratification, and more sophisticated multivariate techniques. If a reader cannot explain away study results on the basis of selection, information, or confounding bias, then chance might be another explanation. Chance should be examined last, however, since these biases can account for highly significant, though bogus results. Differentiation between spurious, indirect, and causal associations can be difficult. Criteria such as temporal sequence, strength and consistency of an association, and evidence of a dose-response effect lend support to a causal link.

Abortion and breast cancer: a hard decision made harder

Over recent years, concerns have been raised about a possible causal relation between induced abortion and subsequent breast cancer. The abrupt hormonal changes associated with termination of pregnancy may induce changes in breast epithelial cells at a stage when they are not fully differentiated and therefore more vulnerable to later development of breast cancer. This review examines the published evidence supporting and refuting this hypothesis and concludes that there are, to date, insufficient data to justify warning women of future breast-cancer risk when counselling them about abortion.

Abortion history and breast cancer risk: results from the Shanghai Breast Cancer Study

Studies of the association between induced abortion and breast cancer risk have been inconsistent, perhaps due to underreporting of abortions. Induced abortion is a well-accepted family planning procedure in China, and women who have several induced abortions do not feel stigmatized. The authors used data from a population-based case-control study of breast cancer among women age 25-64 conducted between 1996 and 1998 in urban Shanghai to assess whether a history of and the number of induced abortions were related to breast cancer risk. In-person interviews were completed with 1,459 incident breast cancer cases ascertained through a population-based cancer registry, and 1,556 controls randomly selected from the general population in Shanghai (with respective response rates of 91% and 90%). After adjusting for confounding, there was no relation between ever having had an induced abortion and breast cancer (odds ratio [OR] = 0.9, 95% confidence interval [CI] 0.7-1.2). Women who had 3 or more induced abortions were not at increased risk of premenopausal breast cancer (OR = 0.9, 95% CI 0.6-1.4) or postmenopausal breast cancer (OR = 1.3, 95% CI 0.8-2.3). These results suggest that a history of several induced abortions has little influence on breast cancer risk in Chinese women.

Induced abortion in relation to breast cancer among parous women: a birth certificate registry study

We wished to assess the relation of induced abortion to the subsequent incidence of breast cancer among parous women, using a design that would prevent the possibility of differentially complete reporting of abortion history by women with breast cancer and controls. Our study was conducted within a cohort of women who gave birth to a child during 1984-1994 while residing in 13 counties of western Washington. Cases were women from the cohort diagnosed with breast cancer between 1984 and 1994. From the remaining cohort members, five controls were matched to each woman with breast cancer by year of index birth (ie, the last child born before breast cancer diagnosis) and by age at delivery. We categorized 463 cases and 2,201 controls according to history of induced abortion as recorded on the index birth certificate. The risk of breast cancer was not found to be associated with a prior induced abortion (estimated relative risk (RR) = 0.9, 95% confidence interval (CI) 0.7-1.2). These results suggest that an induced abortion, if followed at some later time by pregnancy and childbirth, does not increase a woman's risk of breast cancer.

Induced abortion and breast cancer risk

Results from case-control studies suggest that induced abortion may be associated with a small increase in risk of breast cancer. While risk estimates from cohort studies have generally not observed such an association, these studies have had limited information regarding abortion and possible confounding variables. Therefore, we conducted a study among a cohort of post-menopausal women from whom detailed information regarding pregnancy outcomes as well as risk factors for breast cancer had been collected. The study sample included 37,247 Iowa Women's Health Study participants, 55-64 years of age at baseline in 1986, who reported no history of breast, or other, cancer (except non-melanoma skin cancer), and for whom information regarding pregnancy outcomes (that is, live birth, stillbirth, spontaneous abortion, ectopic pregnancy or induced abortion) was available. We used linkage with records of the State Health Registry of Iowa, part of the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) program, to estimate the incidence of breast cancer among cohort members through 1995. We calculated age-adjusted relative risks and 95% confidence intervals using Cox proportional hazards regression. Only 653 women (1.8%) reported an induced abortion. The age-adjusted relative risk of breast cancer among women with prior induced abortion compared with those without was 1.1 (95% CI = 0.8-1.6). Relative risks were higher among women whose age at first abortion was less than 20 or at least 30 years, for those whose abortion took place after their first birth or who never gave birth, and for those with early termination (0-2 months). These estimates varied from 1.3-1.7, but the confidence intervals around each were wide. Since most women in this cohort were beyond their reproductive years when abortion became legal in 1973, the low prevalence of induced abortion argues for a cautious interpretation.

Adolescent reproductive events and subsequent breast cancer risk

OBJECTIVES

This study investigated the relationship between reproductive events during adolescence and subsequent breast cancer risk.

METHODS

Logistic regression models used self-reported data from 862 case patients and 790 controls in the Carolina Breast Cancer Study.

RESULTS

Miscarriage, induced abortion, and full-term pregnancy before 20 years of age were not associated with breast cancer. Among premenopausal women, breast-feeding before 20 years of age was inversely associated with disease. Oral contraceptive use before 18 years of age was positively associated with disease risk among African American women only.

CONCLUSIONS

Pregnancy during adolescence does not appear to influence breast cancer risk, but breast-feeding may. A possible increased breast cancer risk among African American women who used oral contraceptives as adolescents warrants further study.

The alleged association between induced abortion and risk of breast cancer: biology or bias?

The alleged association between induced abortion and breast cancer is one of the most controversial and important questions in women's health today. To help clinicians provide appropriate counseling, we reviewed both the primary data and review articles on this topic. We identified the studies and reviews by using Medline and the reference lists of articles and texts. We then used the U.S. Preventive Services Task Force rating system to evaluate the evidence. Many case-control studies have addressed this question, but their results have been inconsistent. Persistent problems in the case-control studies include selection of an appropriate control group, recall bias (under-reporting of induced abortion by controls), and confounding by other risk factors. Two recent, large cohort studies, which are less susceptible to bias, showed either protection or no effect on breast cancer risk from an induced abortion. At present, level II-2 evidence (cohort and case-control studies) supports a class B recommendation (fair evidence) that induced abortion does not increase a woman's risk of breast cancer later in life.

Accuracy of self-report of breast implants

The accuracy of self-report of breast implants was analyzed using a random sample of 130 of 850 available records from a retrospective cohort study of women who underwent cosmetic surgical procedures from 1963 to 1985. Women with breast implants correctly reported having the surgery 89.3 percent of the time, whereas women having other cosmetic surgical procedures correctly reported having the index surgery 92.7 percent of the time. Younger age at surgery, younger age at survey response, and higher level of education were significantly associated with higher sensitivity of self-report. Despite high validity of reporting of surgery, accuracy of self-report of time of surgery was low, with only 9 percent of women who reported their breast implant surgery providing the correct month and year of surgery. These results are in contrast with a previous validity study.

Induced abortion and the risk of breast cancer

BACKGROUND

It has been hypothesized that an interrupted pregnancy might increase a woman's risk of breast cancer because breast cells could proliferate without the later protective effect of differentiation.

METHODS

We established a population-based cohort with information on parity and vital status consisting of all Danish women born from April 1, 1935, through March 31, 1978. Through linkage with the National Registry of Induced Abortions, information on the number and dates of induced abortions among those women was combined with information on the gestational age of each aborted fetus. All new cases of breast cancer were identified through linkage with the Danish Cancer Registry.

RESULTS

In the cohort of 1.5 million women (28.5 million person-years), we identified 370,715 induced abortions among 280,965 women (2.7 million person-years) and 10,246 women with breast cancer. After adjustment for known risk factors, induced abortion was not associated with an increased risk of breast cancer (relative risk, 1.00; 95 percent confidence interval, 0.94 to 1.06). No increases in risk were found in subgroups defined according to age at abortion, parity, time since abortion, or age at diagnosis of breast cancer. The relative risk of breast cancer increased with increasing gestational age of the fetus at the time of the most recent induced abortion: <7 weeks, 0.81 (95 percent confidence interval, 0.58 to 1.13); 7 to 8 weeks, 1.01 (0.89 to 1.14); 9 to 10 weeks, 1.00 >12 weeks, 1.38 (1.00 to 1.90) (reference category, 9 to 10 weeks).

CONCLUSIONS

Induced abortions have no overall effect on the risk of breast cancer.

Induced abortion as an independent risk factor for breast cancer: a comprehensive review and meta-analysis

STUDY OBJECTIVE

To ascertain, from the published reports to date, whether or not a significantly increased risk of breast cancer is specifically attributable to a history of induced abortion, independent of spontaneous abortion and age at first full term pregnancy (or first live birth); to establish the relative magnitude of such risk increase as may be found, and to ascertain and quantify such risk increases as may pertain to particular subpopulations of women exposed to induced abortion; in particular, nulliparous women and parous women exposed before compared with after the first full term pregnancy.

INCLUDED STUDIES

The meta-analysis includes all 28 published reports which include specific data on induced abortion and breast cancer incidence. Since some study data are presented in more than one report, the 28 reports were determined to constitute 23 independent studies. Overall induced abortion odds ratios and odds ratios for the different subpopulations were calculated using an average weighted according to the inverse of the variance. An overall unweighted average was also computed for comparison. No quality criteria were imposed, but a narrative review of all included studies is presented for the reader's use in assessing the quality of individual studies. EXCLUDED STUDIES: All 33 published reports including data on abortion and breast cancer incidence but either pertaining only to spontaneous abortion or to abortion without specification as to whether it was induced or spontaneous. These studies are listed for the reader's information.

RESULTS

The overall odds ratio (for any induced abortion exposure; n = 21 studies) was 1.3 (95% confidence interval of 1.2, 1.4). For comparison, the unweighted overall odds ratio was 1.4 (1.3,1.6). The odds ratio for nulliparous women was 1.3 (1.0,1.6), that for abortion before the first term pregnancy in parous women was 1.5 (1.2,1.8), and that for abortion after the first term pregnancy was 1.3 (1.1,1.5).

CONCLUSIONS

The results support the inclusion of induced abortion among significant independent risk factors for breast cancer, regardless of parity or timing of abortion relative to the first term pregnancy. Although the increase in risk was relatively low, the high incidence of both breast cancer and induced abortion suggest a substantial impact of thousands of excess cases per year currently, and a potentially much greater impact in the next century, as the first cohort of women exposed to legal induced abortion continues to age.

Familial risk, abortion and their interactive effect on the risk of breast cancer--a combined analysis of six case-control studies

In a previous study in France, we reported that the relative risk of breast cancer associated with a family history of breast cancer was higher in those subjects with a history of abortions. The present study was undertaken to check the existence of this interaction in other studies and to investigate whether the interaction is modified by the time at which abortions occur. Data were obtained from six case-control studies in France, Australia and Russia, with information on family history of breast cancer and abortion for 2693 breast cancer cases and 3493 controls. The interaction effect was estimated in each study separately, then combined using a multivariate weighted average. The relative risk conferred by a family history of breast cancer increased with the number of abortions (1.8 for no abortion, 1.9 for one abortion, 2.8 for two or more). There was a significant interaction between total number of abortions and family history (P = 0.04), but this was no longer significant when adjusted for other risk factors. The familial risk was highest for those who had had an abortion before first childbirth (1.9 for abortion after first childbirth, 2.7 for abortion before first childbirth). The adjusted risk associated with family history was significantly higher in those with an abortion before first childbirth (P = 0.04). Our findings suggest a synergism between familial factors and abortion. The interaction was not substantially modified by the type of abortion (spontaneous or induced) but was modified by the time at which it occurred in relation to first childbirth. This suggests an effect of abortion itself rather than predisposition to abortion. Further studies of breast cancer cases, particularly among BRCA1 gene carriers and their families, could improve our understanding of this effect.