Induced abortion and breast cancer risk
By Joel Brind, Ph.D.,

In a recent case-control study in this journal (1), Mahue-Giangreco et al.
report no increased risk of breast cancer for women with a history of
induced abortion (multivariate OR = 1.05, 95% CI: 0.75-1.48, and OR =
0.69, 95% CI: 0.46-1.04 for parous and nulliparous women, respectively).
In putting their results in the context of the extant literature, the
authors emphasize what they considered to be "controversial results". They
remark: "Some studies reflect an increase in risk associated with induced
abortion, whereas others report no increase in risk".

However, as we pointed out in our 1996 review (2), the record is far more
consistent. In studies on American women, that of Mahue-Giangreco et al.
is actually only the third out of 16 which does not report a positive
association between induced abortion and breast cancer. In fact, 8 of the
other 15 reported a positive association at least on the border of
statistical significance. None has reported a significant negative
association (2).

It is therefore reasonable to ask why the study of Mahue-Giangreco et al.
differs from most other studies on American women. The answer may lie in
another characteristic which sets the Mahue-Giangreco study apart from all
the others, that is, the low participation rate of eligible patients. The
authors report a patient participation rate of 76.8%, which is lower than
most. But importantly, fully 11.5% of eligible patients were not included
because of "patient refusal". This is the highest patient refusal rate of
any study reporting on induced abortion and breast cancer. This high rate
of patient refusal may reflect the sensitive personal nature of induced
abortion, and the consequent reluctance of women to report it. It has been
argued that response bias, in particular, a greater tendency to
underreport abortions among controls compared to patients, could explain
the consistent finding of a positive association (3). However, solid
evidence of such response bias has never been reported. On the contrary,
the lack of such bias in abortion-breast cancer research has been
repeatedly demonstrated (4-6).

It seems more reasonable to suggest that women who are reluctant to
disclose a history of induced abortion would be more likely to opt out of
a study in the first place, rather than agree to provide sensitive
information and then withhold it. In the Mahue-Giangreco study, the
eligible patients constituted a fixed pool (N = 969). Therefore, the
prevalence of induced abortion among the resulting participating patient
population (N = 744) is likely to be underestimated since the refusal rate
is so high. The control group, however, is not nearly as sensitive to such
error, because the pool of eligible controls is open-ended. Specifically,
eligible controls were identified and recruited through a predefined
neighborhood walk. Any eligible controls who refused to participate were
simply replaced by recruiting others from the neighborhood. Therefore, if
abortion-positive patients were substantially over-represented in the
large group of patients refusing to participate, the OR might be
substantially underestimated due to the resulting selection bias. Were
this the case, one would also expect that the overall prevalence of
induced abortion among patients in the study would be lower than expected.

In their discussion, Mahue-Giangreco report that the overall prevalence of
induced abortion among their cases "is consistent with" that of Daling et
al. (5) (Mahue-Giangreco et al. report theirs as 25.8%, compared to 24.9%
for Daling et al., although the correct figures are actually 30.7% and
30.5%, respectively.) Since the women in the Daling et al. study were
within the same age range, and the two studies were "conducted within
approximately the same time frame", Mahue-Giangreco conclude that a
similar prevalence of induced abortion should be expected. In fact, they
also note, as a discrepancy, that the prevalence of induced abortion among
their own controls is approximately 30% higher than that among the Daling
et al. controls.

However, Mahue-Giangreco did not take into consideration an important
demographic difference between the California women they studied, and the
women of Washington state, whom Daling et al. (5) studied. In fact, during
the period 1973-1983, when most of the induced abortions in both studies
took place, the average induced abortion rate was 32% higher in California
than in Washington (7). Hence, the 30% higher prevalence of induced
abortion among the controls in the Mahue-Giangreco study v. the Daling
study, is not discrepant, but rather, to be expected. On the other hand,
the approximately equal prevalence among the patients of the two studies
actually suggests that the prevalence among the California patients was
underestimated, and should have been approximately 30% higher. In absolute
numbers, the expected number of patients in the Mahue-Giangreco study
should thus be approximately 250, instead of the observed 192. This
difference would have raised the observed overall OR to approximately 1.2,
and it could easily have occurred in the context of the 111 patients who
refused to participate. An overall OR of 1.2 would be in line with the
typical observation of a weak positive association between induced
abortion and breast cancer.

Joel Brind, Ph.D., President,
Breast Cancer Prevention Institute
9 Vassar Street
Poughkeepsie, NY 12601 USA

References

1. Mahue-Giangreco, M., Ursin, G., Sullivan-Halley, J., and Bernstein, L.
Induced abortion, miscarriage, and breast cancer risk of young women.
Cancer Epidemiol., Biomark. Prev., 12: 209-14, 2003.

2. Brind, J., Chinchilli, V. M., Severs, W. B., and Summy-Long, J. Induced
abortion as an independent risk factor for breast cancer: a comprehensive
review and meta-analysis. J. Epidemiol. Community Health, 50: 481-496,
1996.

3. Lindefors-Harris, B-M., Eklund, G., Adami, H-O., and Meirik, O.
Response bias in a case-control study: analysis utilizing comparative data
concerning legal abortions from two independent Swedish studies. Am. J.
Epidemiol., 134: 1003-8, 1991.

4. Howe, H. L., Senie, R. T., Bzduch, H., and Herzfeld, P. Early abortion
and breast cancer risk among women under age 40. Int J Epidemiol., 18:
300-304, 1989.

5. Daling, J. R., Malone, K. E., Voigt, L. F., White, E., and Weiss, N. S.
Risk of breast cancer among young women: relationship to induced abortion. J Natl Cancer Inst 86: 1584-1592, 1994.

6. Tang, M-T. C., Weiss, N. S., Daling, J. R., and Malone, K. E.
Case-control differences in the reliability of reporting a history of
induced abortion. Am. J. Epidemiol., 151: 1139-1143, 2000.

7. State-by-state trends in abortion in the United States. The Alan
Guttmacher Institute. www.guttmacher.org/sections/abortion.html, 2003