Memorandum 26
Submission from Cape Hill Medical Centre
ABORTION AND
BREAST CANCER:
A CRITICAL REVIEW
scientific developments relating to the abortion
act 1967. Responding in particular to paragraphs 2(a) and 3
SUMMARY
Breast cancer rates have been
rising in Europe and North America for several decades and are
projected to rise further.1
Oestrogen as well as promoting
growth of healthy breast tissue is also recognized as a carcinogen.2
Any factor which increases the
development of cancer prone type 1 breast lobules at the expense
of cancer resistant type 3 breast lobules could also increase
a woman's long term risk of developing breast cancer.3
As well as the undisputed effect
of delay in first full term pregnancy being linked to breast cancer
there is significant epidemiological evidence for an independent
causal association between induced abortion and breast cancer;
a majority of studies showing an association with no proven evidence
of response bias; 17 statistically significant studies; observation
of a dose effect;4 concordance of exposure with effect; a biologically
plausible explanation;5 and experimental studies.6
There are significant problems
with the methodology of most of the recent prospective studies.
It should be routine practice
for anyone involved in counseling a woman considering an abortion,
to provide as much information about possible risks of breast
cancer pertaining to her decision as a reasonable patient would
wish to know.
INTRODUCTION
1. The age standardized incidence of breast
cancer in the UK is among the highest in the world.7 The disease
is the commonest cancer in Europe and the commonest cause of cancer
death in women in Europe.8 Any factor which potentially affects
this high incidence needs careful examination. Oestrogen overexposure
is associated with breast cancer and oestrogen is a promoter of
both normal and abnormal breast tissue.5 Induced abortion in animals
is associated with mammary cancer6 and there are concerns that
the same risks might accrue to women undergoing induced abortion.
There are a majority of studies which show an increased risk,
17 of which are statistically significant on their own.
Age standardised (European) incidence and mortality
Rates, female breast cancer. GB 1975-2004
Year of diagnosis/death
2. Various guidelines and position papers
have been published9, 10 which have suggested that there is no
independent association between induced abortion and future risk
of breast cancer. There is no dispute that delayed childbearing
per se enhances breast cancer risk. 11 What is at issue is whether
there might be (at least for some women) an additional risk conveyed
by induced abortion.
PHYSIOLOGY
3. The fully mature breast at the end of
puberty contains mostly type one lobules which are incapable of
producing milk. These type one lobules are more susceptible in
later life to malignant change. Towards the end of pregnancy,
type one lobules mature into milk producing type three lobules.
The type three lobules are more cancer resistant and account for
the majority of breast lobules in the post partum woman.
A full term pregnancy therefore decreases life time risk of breast
cancer, and oestrogen exposure before first full term pregnancy
increases risk. Any factor which increases a woman's exposure
to oestrogen surges before she has had the opportunity for the
development of type three breast lobules potentially increases
long term breast cancer risk.
RESPONSE BIAS
THEORY
4. Both the American College of Obstetricians
and Gynecologists and the Royal College of Obstetricians and Gynaecologists
in the UK have cited studies to support the position that doubt
should be cast on the validity of case-control studies of abortion
and breast cancer because of the possibility of response bias.
5. In the first of these two studies which
look at women with histologically proven breast cancer, the authors
provide figures comparing reported histories of abortion, with
data on the Swedish national abortion registry. 12 They claim
to have found evidence of abortion over reporting in women with
breast cancer compared to controls, and underreporting of abortion
in healthy women. Seven women with breast cancer claimed to have
had an abortion when the computerized registry could find no record
of this. The researchers made the daring assumption that the women
were lying about having abortions they never had.
6. Ironically, the authors of the second
paper gave the following comment on the Swedish study saying that
"It would be highly unlikely for women to report an induced
abortion that never took place, which shows that the registry
was not complete." 13 Similar thoughts were expressed by
Daling et al, "We believe that it is reasonable to
assume that virtually no women who truly did not have an abortion
would claim to have had one." Similarly, "These results
argue against there being any substantial difference in the accuracy
of reporting of induced abortions by women with cancer and women
without cancer." 14
7. Eventually in 1998 Meirik, Adami and
Eklund admitted that the Swedish abortion register was in fact
not complete15 thus invalidating their figures.
8. In the second paper, Rookus and Van Leeuwen
also claim underreporting of abortions in healthy control subjects.
16 Their evidence relied on a small group of parous women under
age 45 containing only 13 subjects.
9. It remains to be argued why the working
hypothesis would be that women with breast cancer are more likely
to admit to having had abortions than women who are healthy? It
would be equallyif not moreplausible to argue that
women with breast cancer might be more likely to deny factors
such as a history of abortion which may have contributed to their
illness. In fact, it has been shown that in cases of cervical
cancer, which is known not to be linked with abortion, no increase
in the rate of reporting previous abortion was noted.
10. Researchers have tested for recall bias
in women after abortion and found no credible evidence for it.
14, 17-19 The 2000 study by Tang et al concludes, "The
authors data do not suggest that controls are more reluctant to
report a history of induced abortion than are women with breast
cancer".18
DATA FROM
CONTROVERSIAL STUDIES
11. The RCOG 2004 guidelines refer to "the
most methodologically rigorous of the previously reviewed studies
... those based on linkage of registry data" and the ACOG
paper9 cites two of the most commonly referenced ones. 20, 21
12. The first of these looked at women born
in Denmark between 1935 and 1978. 20 This study made the following
errors:
The study contains a population,
some of whom were as old as 38 when the computerized abortion
registry began. This indicates a skewed distribution towards older
post abortion patients in the study.
In the study, over 50% of the
women with prior abortions were recorded as having had the abortion
over 35 years of age. Yet we know from Danish government statistics
that the median age of abortion is 27.
The year of legalization of
abortion in Denmark was stated in the paper to be 1973 whereas
it was 1938.
Abortions in Denmark were hand
recorded from 1940 onwards although this fact is not mentioned
in the paper. This means that up to 1973 about 60,000 women listed
in the study cohort as not having had abortions in fact did have
them.
The average follow-up time for
abortion-negative women was 21 years, whereas for abortion-positive
women it was nine years.
13. So the older women, who are more likely
to suffer from breast cancer, were counted as having no history
of induced abortion resulting in significant underreporting of
abortion history. 22, 23
14. Melbye et al also make adjustments
for the birth cohort effect. This is the trend seen in birth cohorts
of an increasing incidence in breast cancer after adjusting for
age. If abortion is a significant reason why the incidence of
breast cancer rose for most of the twentieth century, adjusting
for the birth cohort effect eliminates the effect of the factor
under study (abortion) and almost guarantees the nil result. Even
allowing for these errors, Melbye et al still showed a 3% increased
risk in breast cancer for each week of gestation, so that after
18 weeks an abortion would be associated with an 89% increase
in risk.
15. In the Swedish register study, 41% of
the cohort of breast cancer patients were nulliparous at the time
of the index abortion. 21 This compared with a 49% nulliparity
rate in the control group. Since nulliparity is associated with
an increased risk of breast cancer the lack of correction for
this in the study led to an underestimation of risk in the group
exposed to abortion.
PRE-TERM
DELIVERY AND
BREAST CANCER
16. In 1999, Melbye and colleagues made
corrections to errors to their earlier paper.4 Here they reported
a 2.11 times increased risk in breast cancer among women who had
pre-term delivery prior to 32 weeks gestation. An association
between pre-term birth and later onset of breast cancer was also
reported from Sweden in the same year, particularly with older
women. 24 These results fit with the animal models of Russo and
Russo6 and also give a further clue as to why abortion may be
associated with subsequent increased breast cancer risk. It is
now known that induced abortion is itself associated with an increased
risk of subsequent pre-term delivery. 25, 26
DATA OFTEN
OMITTED FROM
GUIDELINES
17. Daling's study in 1994 showed an increased
risk of breast cancer of 50% in aborted women overall. In addition,
it identified various subgroups in which the risk was higher still.
There was a relative risk of 2.5 in women who had abortions below
the age of 18. In a subgroup of teenagers with a positive family
history of breast cancer and who had undergone abortions, every
woman had developed breast cancer by the age of 45. 14
18. It is interesting that the ACOG and
RCOG guidelines groups fail to mention the 1989 prospective study
by Howe et al. 29 This study showed a 90% increase in risk
of breast cancer among women after abortion. There are now six
studies which show a relative risk of greater than two, 27-32
and over 20 others showing an association between abortion and
breast cancer. 14, 16, 17, 27, 33-52 Over half of these are statistically
significant on their own. Brind and colleagues in their quantitative
meta-analysis in 1996 found an odds ratio of breast cancer following
abortion to be 1.3, with a ratio of 1.5 before the first term
pregnancy. 25 In the review by Wingo et al53 published
in the following year it is clear that a trend to increased risk
of breast cancer exists after abortion. Particularly striking
is the data on nulliparous women. There are nine such studies
cited14, 17, 21, 35, 37, 38, 45, 50, 54 and seven of the nine
show a positive association. 14, 21, 35, 37, 38, 45, 50
ACTUARIAL DATA
19. More evidence of an association is the
study by Carroll1 using data from the Office of National Statistics.
Breast cancer rates have been rising despite a decrease in women's
age at first birth. However, nulliparous abortion rates show a
marked increase in cohorts born during and since the 1940's. Since
the majority of women now having abortions in the UK are nulliparous,
Carroll estimates that up to 35% of new breast cancer cases could
be attributable to abortion carried out perhaps decades previously.
When the additionally carcinogenic effect of nulliparous abortions
are taken into account, the figure rises to 52%. Projections from
this data to the year 2023 suggest an eventual incidence of breast
cancer of around 77,000 cases per year in the UK.
MORE RECENT
EVIDENCE
20. Since the year 2000 a number of studies
have been published using prospective data and several have been
cited by guidelines committees as evidence for a null association.
21. A study published by the Unit of Health-Care
Epidemiology at Oxford University found a slightly lower than
expected rate of abortion before breast cancer. 55 The authors,
however, admitted a weakness in their data since the study by
Goldacre et al found an incidence rate of induced abortion
in the group of only 6.7% of the national incidence rate. 56
22. Two studies19, 57 on Chinese women have
been published in addition to the 1995 abstract by Bu et al.
23 Both of these reported no or only slightly increased risk.
Both studies acknowledged that the overwhelming majority of abortions
in China take place after a live birth. 58 So, in a country with
very high levels of abortion which make controls difficult to
find, breast-cancer rates are not increasing for women who are
mostly parous. This is in contrast with the UK where most abortions
are performed before first full term pregnancy.
23. In Sweden a 2003 study reported a reduction
in breast cancer risk with previous induced abortion. 59 This
study was limited to pre-menopausal women. There has been a 45%
increased incidence in breast cancer in Sweden in women aged 50-69
attributed to the introduction of screening but the expected fall
in incidence after age 69 has failed to materialise.60 Like the
other two record linkage studies mentioned, 20, 55 substantial
amounts of data appear to be missing. 61
24. Other problems with cohort studies include:
inadequate follow up time due
to overlap of exposure and diagnostic periods20, 63-66 confounded
by the transient effect of full term pregnancy; 63
missing information. 64, 68
25. The study by Brewster et al in
Scotland 68 is a striking example of an unrepresentative sample
since only 5.6% of its study population were nulliparous prior
to having an abortion whereas 58% of abortions in Scotland are
performed on nulliparous women.
DELAYED CHILDBEARING
AS A
RISK FACTOR
26. The RCOG guidelines10 refer to "a
major systematic review... which lent further support to these
conclusions" arguing against a causal relationship between
induced abortion and breast cancer. 69 The Collaborative Group
on Hormonal Factors in Breast Cancer drew attention to the influence
of parity in their 2002 report. 11 There was a 3% increase risk
of breast cancer for every year of delayed childbearing. The figure
of a 5% increased risk for pre-menopausal breast cancer and 3%
increased risk for post-menopausal breast cancer for each year
of delayed first full term pregnancy was similarly derived from
an extensive literature review. 70
27. In Slovenia, a country which has not
had much breast cancer screening, breast cancer incidence increased
by 70% from 1971 to 1993. These changes were dominated by cohort
effects, with the percentage of nulliparous women in the cohort
the most important reproductive variable. 71
28. Induced abortion as a cause of delayed
childbearing should therefore be considered a risk factor for
breast cancer. In the Collaborative Group paper of March 2004
an argument was presented that compared prospective data with
retrospective data. The authors concluded that the retrospective
data are likely to be flawed because of "response bias"
but failed to provide any credible evidence for this. Beral cites
the study by Lindefors-Harris12 but fails to mention that the
authors of this study retracted their claim of abortion over-reporting
in 1998 as mentioned above.
29. A thorough analysis of the Lancet paper69
has been published by the Breast Cancer Prevention Institute.
72 Some of the more obvious errors are the following:
(i) Included are the prospective studies
with substantially incomplete data. 4, 20, 55, 59
(ii) Excluded in the "totality of the
worldwide epidemiological evidence" were 12 studies16, 29-31,
34, 37, 39, 44, 45, 47, 49, 50 for inappropriate reasons such
as:
(a) "Principal investigators of four
studies could not be traced",30, 37, 49, 50
(b) "original data could not be retrieved
by the principal investigators",34, 39
(c) "researchers declined to take part
in the collaboration",44, 45
(iii) Three studies were not even mentioned.
27, 28 These studies between them account for an 80% increased
risk and 10 of the 15 are statistically significant alone.
30. As noted above, most of the studies
which showed significant elevations in risk with induced abortion
were inappropriately excluded from the analysis. Then, by combining
certain groups of studies under the title of "other",
it is made to look as if no study ever found a relative risk greater
than 1.4. The reality is that six studies have reported overall
relative risks greater than 2.0. 28-32, 73
31. The heading in the summary chart of
the Lancet paper is misleading: "Relative risk of breast
cancer, comparing the effects of having had a pregnancy that ended
as an induced abortion versus effects of never having had that
pregnancy." This is an inappropriate comparison; the proper
one should be comparing women who have abortions with women who
continue with the pregnancy.
MEDICOLEGAL CONSIDERATIONS
32. A doctor's duty to explain risk to a
patient extends to all risks which a reasonable doctor would disclose
or those risks which a reasonable patient would attach significance.
The state of Texas under the Woman's Right to Know Act now mandates
that abortion providers tell women considering abortion about
all possible complicationsboth immediate and long termfollowing
abortion. Other US states have adopted similar legislation.
33. Kindley discusses the evidence from
a lawyer's perspective and discusses what information a reasonable
patient would consider material to her decision. 74 He concludes
that information regarding the abortion breast cancer link is
likely to be significant in the decision making process of a reasonable
patient and therefore must be disclosed to women considering abortion.
There have been three successful prosecutions of "failure
to warn" cases brought in the US and in Australia and in
the UK, The Chester v Afshar decision of 2004 swept aside
the requirement of "causation" in "failure to warn"
cases. 75
34. The abortion provider or abortion counselor
may therefore be negligent by not informing women (especially
nulliparous women) of a possible long term increased risk of breast
cancer following induced abortion.
RECOMMENDATIONS
35. Women seeking advice or counseling regarding
abortion are vulnerable and cannot make an informed decision without
adequate information. Abortion providers are rarely neutral and
women need more, and better quality information regarding long
term risk of breast cancer whether they choose abortion or full
term pregnancy.
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